U.S. patent application number 11/551458 was filed with the patent office on 2007-02-22 for method and apparatus for non-invasive monitoring of blood substances using self-sampled tears.
This patent application is currently assigned to EYELAB GROUP, LLC. Invention is credited to Geun Sig Cha, Bruce E. Cohan, Gang Cui, Zvi Flanders, Donald E. Gillespie, Jong Sik Kim, Mark E. Meyerhoff, Hakhyun Nam.
Application Number | 20070043283 11/551458 |
Document ID | / |
Family ID | 29250543 |
Filed Date | 2007-02-22 |
United States Patent
Application |
20070043283 |
Kind Code |
A1 |
Cohan; Bruce E. ; et
al. |
February 22, 2007 |
Method and Apparatus for Non-Invasive Monitoring of Blood
Substances Using Self-Sampled Tears
Abstract
A method and apparatus for non-invasively determining the
concentration of a substance in blood, such as glucose, include a
sample portion arranged for contacting an eye region of a user to
obtain a tear fluid sample, a sensor in communication with the
sample portion for generating a signal related to the tear
substance concentration, and a processor in communication with the
sensor for determining a blood substance concentration
corresponding to the tear substance concentration.
Inventors: |
Cohan; Bruce E.; (Ann Arbor,
MI) ; Cha; Geun Sig; (Seoul, KR) ; Meyerhoff;
Mark E.; (Ann Arbor, MI) ; Nam; Hakhyun;
(Seoul, KR) ; Gillespie; Donald E.; (Ann Arbor,
MI) ; Cui; Gang; (Jilin, CN) ; Kim; Jong
Sik; (Seoul, KR) ; Flanders; Zvi; (Ann Arbor,
MI) |
Correspondence
Address: |
BROOKS KUSHMAN P.C.
1000 TOWN CENTER
TWENTY-SECOND FLOOR
SOUTHFIELD
MI
48075
US
|
Assignee: |
EYELAB GROUP, LLC
Ann Arbor
MI
|
Family ID: |
29250543 |
Appl. No.: |
11/551458 |
Filed: |
October 20, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10404702 |
Apr 1, 2003 |
7133712 |
|
|
11551458 |
Oct 20, 2006 |
|
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|
60370552 |
Apr 5, 2002 |
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|
Current U.S.
Class: |
600/345 ;
600/347; 600/365; 600/573; 600/584 |
Current CPC
Class: |
A61B 2562/0295 20130101;
G01N 2001/1056 20130101; A61B 5/1486 20130101; C12Q 1/32 20130101;
Y10T 436/144444 20150115; G01N 2035/0465 20130101; G01N 33/528
20130101; C12Q 1/54 20130101; G01N 2001/149 20130101; G01N 27/3272
20130101; C12Q 1/006 20130101 |
Class at
Publication: |
600/345 ;
600/347; 600/365; 600/573; 600/584 |
International
Class: |
A61B 5/05 20060101
A61B005/05; A61B 5/00 20060101 A61B005/00 |
Claims
1. An apparatus for determining the concentration of a substance in
blood, the apparatus comprising: a housing; a test probe arranged
to be received within the housing, the test probe arranged to
engage an eye region of a user to obtain a tear fluid sample while
the test probe is received within the housing; a sensor disposed
within the housing in communication with the test probe for
generating a signal related to the tear substance concentration;
and a processor disposed within the housing in communication with
the sensor for determining a blood substance concentration
corresponding to the tear substance concentration.
2. The apparatus according to claim 1, wherein the substance
includes glucose.
3. The apparatus according to claim 2, wherein the sensor is
operable to detect glucose concentrations less than about 20 mg/dL
with a resolution of at least 5 mg/dL.
4. The apparatus according to claim 1, wherein the test probe is
arranged to allow self-sampling of the tear fluid sample by a
user.
5. The apparatus according to claim 1, wherein the test probe
includes an inlet extending outwardly therefrom.
6. The apparatus according to claim 5, wherein the inlet includes a
capillary member or a wicking membrane.
7. The apparatus according to claim 1, wherein a volume of the tear
fluid sample held by the test probe is less than about 0.5
.mu.L.
8. The apparatus according to claim 1, wherein the test probe is
removable from the housing.
9. The apparatus according to claim 1, further comprising a display
screen in communication with the processor for displaying the blood
substance concentration.
10. The apparatus according to claim 1, further comprising memory
in communication with the processor for storing the blood substance
concentration.
11. The apparatus according to claim 1, wherein the housing is
generally pen-shaped.
12. The apparatus according to claim 1, wherein the test probe
includes an enzyme for reacting with the substance in the tear
fluid sample.
13. The apparatus according to claim 12, wherein the enzyme
includes glucose dehydrogenase.
14. The apparatus according to claim 12, wherein the test probe
includes an electrode system provided therein, the apparatus
further comprising a power source for applying a voltage to the
electrode system to induce an electrochemical reaction of the
enzyme and the substance, and wherein the sensor detects a current
resulting from the electrochemical reaction and the processor
determines a tear substance concentration from the detected
current.
15. The apparatus according to claim 1, wherein the apparatus is
configured to allow the electronic transfer of substance
concentration data.
16. An apparatus for determining glucose concentration in a tear
fluid sample, the apparatus comprising: a housing; a test probe
arranged to be received within the housing, the test probe arranged
to engage an eye region of a user to obtain a tear fluid sample
while the test probe is received within the housing; a sensor
disposed within the housing in communication with the test probe
for generating a signal related to the glucose concentration in the
tear fluid sample; and a processor in communication with the sensor
for processing the signal to determine the tear glucose
concentration.
17. An apparatus for determining glucose concentration in blood,
the apparatus comprising: a housing; a test probe arranged to be
received within the housing, the test probe having an inlet
arranged to engage an eye region of a user to obtain a tear fluid
sample while the test probe is received within the housing, the
test probe having an enzyme provided therein for initiating a
reaction with the tear fluid sample, wherein a volume of the tear
fluid sample held by the test probe is less than about 0.5 .mu.L; a
sensor disposed within the housing in communication with the test
probe for detecting a signal generated by the reaction; a processor
in communication with the sensor for determining a tear glucose
concentration from the detected signal and correlating the
determined tear glucose concentration with a blood glucose
concentration; and means for providing an output indicative of the
blood glucose concentration.
18. The apparatus according to claim 17, wherein the test probe is
arranged to allow self-sampling of the tear fluid sample by a
user.
19. The apparatus according to claim 17, wherein the housing is
generally pen-shaped.
20. The apparatus according to claim 17, wherein the sensor is
operable to detect glucose concentrations less than about 20 mg/dL
with a resolution of at least 5 mg/dL.
21. The apparatus according to claim 17, wherein the enzyme
includes glucose dehydrogenase.
22. The apparatus according to claim 17, wherein the test probe
includes pyrrolo-quinoline-quinone as a coenzyme.
23. The apparatus according to claim 17, wherein the test probe
includes an electron transfer mediator.
24. The apparatus according to claim 23, wherein the electron
transfer mediator includes a ruthenium complex.
25. The apparatus according to claim 23, wherein the test probe
includes a base plate, a cover plate, and a spacer disposed between
and joining the base and cover plates, the base and cover plates
including a converse-type electrode system, the apparatus further
comprising a power source for applying a voltage to the electrode
system to induce an electrochemical reaction of the enzyme and the
electron transfer mediator with glucose in the tear fluid sample
and generate a current related to tear glucose concentration.
26. The apparatus according to claim 25, wherein the processor
determines a tear glucose concentration from the generated current
and multiplies the tear glucose concentration by a calibration
factor to determine the corresponding blood glucose
concentration.
27. The apparatus according to claim 17, further comprising a
speaker in communication with the processor.
28. The apparatus according to claim 17, wherein the apparatus is
configured to allow the electronic transfer of glucose
concentration data.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 10/404,702 filed Apr. 1, 2003 which, in turn, claims the
benefit of U.S. provisional application Ser. No. 60/370,552 filed
Apr. 5, 2002.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to a method and apparatus for
non-invasive monitoring of blood substances, particularly glucose,
using self-sampled tears.
[0004] 2. Background Art
[0005] The measurement of glucose in blood plasma is perhaps the
most important physiologic analyte measurement in medicine, as
diabetes has immense public health implications. Diabetes is a
leading cause of disability and death, affecting approximately
seventeen million Americans. The total annual cost of treating
diabetes and its complications in the United States is in excess of
$150 billion, a large part of the total national expenditure for
health care.
[0006] The medical management of diabetes by tight glycemic (blood
glucose) control can minimize its devastating kidney, ocular,
neurological, and vascular complications, as documented in the
National Institutes of Health-sponsored Diabetes Control and
Complications Trial. However, the trial resulted in a three-fold
increase in hypoglycemic incidents. Of great concern to
diabetologists in their care of these patients is hypoglycemia
awareness because of its serious risk for morbidity and
mortality.
[0007] Tight glycemic control requires frequent measurement by the
patient of his/her blood glucose levels, which typically requires a
"finger stick" to obtain a blood sample up to eight times daily.
This procedure is painful and inconvenient for even the most
compliant patients, such that limited patient compliance with
self-testing is a significant problem in the medical management of
this disease. Accordingly, the need for a non-invasive approach to
diabetes management is universally recognized to achieve the goal
of involving patients in a proactive way in their glycemic control,
both in monitoring blood glucose and in insulin delivery.
[0008] The tremendous need for a reliable, cost-effective method of
non-invasive blood glucose measurement for diabetes management has
stimulated hundreds of analytic approaches. The invasiveness of
these approaches extends from implanted sensors through a range of
less, to minimally, to non-invasive methods. Minimally invasive
methods include chemical or spectroscopic measurement of
interstitial fluid from the skin obtained by reverse iontophoretic,
electroosmotic, or thermal microporation sampling. Among the other
technologically sophisticated approaches to glucose measurement are
spectroscopy (transcutaneous infrared, fluorescence lifetime,
pulsed laser photoacoustic, and far infrared), analysis of breath,
optical measurements of the aqueous humor of the eye, polarimetry,
and radio wave impedance. Some of these methods have worked well in
controlled laboratory testing, but in practice other chemical
species, tissue optics, variations in temperature, and other
factors have confounded the measurement. For all spectroscopic
approaches, the major problem is the need for frequent calibration,
as infrared absorption bands for various chemicals in blood or
interstitial fluid can overlap significantly and are influenced by
temperature and hydrogen bonding effects.
[0009] The concentration of low molecular weight analytes, like
glucose, in blood plasma is correlated with the levels found in
lacrimal fluid, or tears. While a number of methods for measuring
this analyte have been applied to tears, two main factors have
prevented the practical use of measuring tear glucose concentration
as a means for self-monitoring blood glucose concentration: 1) the
low level of glucose in tears, reported in a recent study (see Chen
et al., J Cap Elec 1996; 5:243-248) to be approximately 1/25 the
level in blood, and 2) the small volume of tear fluid as compared
with blood that is readily available for analysis. In the
aforementioned Chen study, glucose concentration in microliter
samples of human tears obtained with capillary tubes was determined
by capillary electrophoresis (CE) with laser-induced fluorescence
(LIF), a sophisticated method limited to research chemistry
laboratories because of its technical complexity.
[0010] Currently, no practical, entirely non-invasive system and
method exists for patients to self-monitor their blood glucose with
the level of accuracy and responsiveness required.
SUMMARY OF THE INVENTION
[0011] Therefore, it is an object according to the present
invention to provide a method and apparatus for determining the
concentration of a substance in tears which will allow for indirect
monitoring of the substance concentration in blood.
[0012] It is a further object according to the present invention to
provide an improved method and apparatus for non-invasively
determining blood glucose concentration in a simple and accurate
manner.
[0013] It is a still further object according to the present
invention to provide a method and apparatus for determining glucose
concentration in tear fluid that is self-sampled by a patient.
[0014] Accordingly, a method is provided for determining the
concentration of a substance in blood, such as glucose, where the
method includes providing a test apparatus having a sample inlet,
and engaging an eye region of a user with the sample inlet to
obtain a tear fluid sample. The method further includes processing
the tear fluid sample using the test apparatus to determine a tear
substance concentration, and correlating the determined tear
substance concentration with a blood substance concentration.
[0015] Correspondingly, an apparatus for determining the
concentration of a substance in blood, such as glucose, is provided
which includes a sample portion arranged for contacting an eye
region of a user to obtain a tear fluid sample, a sensor in
communication with the sample portion for generating a signal
related to the tear substance concentration, and a processor in
communication with the sensor for determining a blood substance
concentration corresponding to the tear substance
concentration.
[0016] In a preferred embodiment, a user self-samples tear fluid
from his/her eye region by engaging a lower lid region and
obtaining tear fluid from a tear meniscus. While obtaining the tear
fluid sample, the eye may be substantially closed. Advantageously,
the tear fluid sample can be less than about 0.5 .mu.L. Preferably,
the sample portion, or test probe, includes an inlet that extends
outwardly from the test probe, such as a capillary member or a
wicking membrane, to facilitate contact with the eye. The sample
portion is preferably removable from the apparatus. In a preferred
embodiment, the apparatus includes a generally pen-shaped
housing.
[0017] The sample portion includes an enzyme, preferably glucose
dehydrogenase, for reacting with the substance in the tear fluid
sample. The sample portion further includes an electron transfer
mediator, such as a ruthenium complex. The coenzyme
pyrrolo-quinoline-quinone (PQQ) can also be utilized. In a
preferred embodiment, the sample portion includes a base plate, a
cover plate, and a spacer disposed between and joining the base and
cover plates, where the base and cover plates include a
converse-type electrode system. A power supply is provided for
applying a voltage to the electrode system to induce an
electrochemical reaction of the enzyme and the electron transfer
mediator with the substance in the tear fluid sample and generate a
current related to the tear substance concentration. The processor
then determines a tear glucose concentration from the generated
current and multiplies the tear glucose concentration by a
calibration factor to determine the corresponding blood glucose
concentration. An amplifier can be provided for amplifying the
generated current, and a speaker can be provided to generate
audible indications for the user. Additionally, a display screen is
provided in communication with the processor for displaying the
blood substance concentration, and memory is provided in
communication with the processor for storing the blood substance
concentration.
[0018] In accordance with the present invention, a method for
determining glucose concentration in a sample of tear fluid
includes providing a test apparatus having a sample inlet arranged
for contacting an eye region of a user, engaging the eye region
with the sample inlet to obtain a tear fluid sample, and processing
the tear fluid sample using the test apparatus to determine the
glucose concentration in the tear fluid sample. Correspondingly, an
apparatus for determining glucose concentration in a tear fluid
sample includes a sample portion arranged for contacting an eye
region of a user to obtain a tear fluid sample, a sensor in
communication with the sample portion for generating a signal
related to the glucose concentration in the tear fluid sample, and
a processor in communication with the sensor for processing the
signal to determine the tear glucose concentration.
[0019] In further accordance with the present invention, a method
for determining blood glucose concentration includes providing a
test apparatus including a sample portion which includes an enzyme.
The method further includes engaging an eye region of a user with
the sample portion to obtain the tear fluid sample, reacting the
tear fluid sample with the enzyme to generate a signal related to
the tear glucose concentration, processing the signal using the
test apparatus to obtain a blood glucose concentration
corresponding to the tear glucose concentration, and providing an
output indicative of the blood glucose concentration.
Correspondingly, an apparatus for determining glucose concentration
in blood includes a sample portion having an inlet arranged for
contacting an eye region of a user to obtain a tear fluid sample,
where the sample portion contains an enzyme for initiating a
reaction with the tear fluid sample. A sensor in communication with
the sample portion detects a signal generated by the reaction, and
a processor in communication with the sensor determines a tear
glucose concentration from the detected signal and correlates the
determined tear glucose concentration with a blood glucose
concentration. The apparatus further includes means for providing
an output indicative of the blood glucose concentration.
[0020] According to the present invention, a probe is provided for
obtaining a tear fluid sample. The probe includes an input end
arranged to contact an eye region of a user to obtain the tear
fluid sample, and a probe body in communication with the input end
and having components for generating a reaction with a substance,
such as glucose, in the tear fluid sample. The probe further
includes an output end in communication with the probe body and
arranged to be removably mated with a test apparatus for
determining a concentration of the substance in the tear fluid
sample.
[0021] In a preferred embodiment, the input end extends outwardly
from the probe body, and can include a capillary member or a
wicking membrane. The probe body preferably includes an enzyme,
such as glucose dehydrogenase, for reacting with the substance in
the tear fluid sample, and can also include a coenzyme, such as
pyrrolo-quinoline-quinone (PQQ). The probe body preferably further
includes an electron transfer mediator, such as a ruthenium
complex. The probe body preferably includes a base plate, a cover
plate, and a spacer disposed between and joining the base and cover
plates, where the base and cover plates include an electrode
system. The electrode system is of a converse type, where a working
electrode is provided on one of the base and cover plates and a
reference electrode is provided on the other of the base and cover
plates.
[0022] The above objects and other objects, features, and
advantages of the present invention are readily apparent from the
following detailed description of the best mode for carrying out
the invention when taken in connection with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIGS. 1a and 1b are photographs of front and side views,
respectively, of a user self-sampling tears according to the
present invention;
[0024] FIG. 2 is a perspective view of a preferred embodiment of
the test apparatus of the present invention, wherein the test probe
is shown in a removed position;
[0025] FIG. 3 is a perspective view of the test probe of FIG.
2;
[0026] FIG. 4 is an exploded perspective view of the test probe of
FIG. 3;
[0027] FIG. 5 is a perspective view of an alternative embodiment of
the test probe according to the present invention;
[0028] FIG. 6 is an exploded perspective view of the test probe of
FIG. 5;
[0029] FIG. 7 is a perspective view of another alternative
embodiment of the test probe according to the present
invention;
[0030] FIG. 8 is an exploded perspective view of the test probe of
FIG. 7;
[0031] FIG. 9 is an illustration of the electrochemical reaction
scheme of the test probe according to a preferred embodiment of the
present invention;
[0032] FIG. 10 is a schematic illustration of the electronic
circuit and components of the test apparatus of the present
invention;
[0033] FIG. 11 is a graph of the dynamic response of current
measured for a range of glucose concentrations in PBS using the
apparatus of the present invention;
[0034] FIG. 12 is a graph of the calibration curve of currents
measured for glucose concentrations from 0-40 mg/dL using the
apparatus of the present invention;
[0035] FIG. 13 is a graph depicting the precision and accuracy for
glucose concentrations ranging from 0-28 mg/dL in a simulated tear
matrix; and
[0036] FIG. 14 is an enlarged portion of the graph of FIG. 13
depicting the precision and accuracy for low concentrations of
glucose ranging from 0-5 mg/dL.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
[0037] The method and apparatus of the present invention provide
for the practical, non-invasive determination of the concentrations
of substances, particularly glucose, in human tears in order to
indirectly monitor the level of this important analyte in blood.
The method and apparatus described herein are designed for the
special limitations of analysis of tear fluid, namely the low
glucose concentration in tears compared with blood and the small
sample volume available. The present invention advances tight
glycemic control of diabetes by permitting users to monitor their
blood glucose levels by self-measuring the glucose levels in their
tears, wherein the tear fluid sample is easily obtained by a user
and accurate results are immediately available.
[0038] By way of background, the primary aqueous component of tears
is secreted by the lacrimal gland, which is located beneath the
outer portion of the upper eyelid. In this gland, a fraction of the
glucose in blood crosses into the tears. This fluid flows from the
gland through a number of tiny lacrimal gland ducts onto the
surface of the eye where it forms a thin layer that maintains a
wet, optically smooth corneal surface, and lubricates with moisture
the conjunctiva, the mucous membrane which covers the sclera and
lines the lids, joining under the upper and lower lids in a cul de
sac. The tear fluid is continually secreted and flows across the
eye at a rate of about 2 .mu.l per minute. The tears form a
meniscus along the lower lid margin and a shallow pool, the
lacrimal lake, between the inner (nasal) edge of the cornea and
near the joining of the lids nasally. The total volume of lacrimal
fluid on the surface of the eye is about 7 .mu.l. A small opening,
the lacrimal punctum, near the nasal end of each lid opens into a
tubular channel, the lacrimal canaliculus, which drains the tears
into the lacrimal sac. From there, the lacrimal fluid empties into
the nose.
[0039] With reference first to FIGS. 1a and 1b, photographs of a
user conducting self-sampling of tear fluid are shown. According to
the present invention, a user self-samples his/her tear fluid as
follows. The user places a sample inlet of a test apparatus 10,
described below with reference to FIGS. 2-8, in contact with
his/her eye region E, preferably at the lower lid margin L. To aid
in this procedure, the user may exert gentle traction with his/her
finger on the skin of the lower lid, as is typically done for
inserting contact lenses, and may view the procedure in a mirror.
Tear fluid is then obtained from the tear meniscus along the lower
lid margin L, and the sample drawn up into the test apparatus 10 by
capillary action without requiring any intermediate handling of
tears. Tears can also be self-sampled from the lateral canthal
region and from the lacrimal lake. It is understood that the test
apparatus 10 need not necessarily engage the eye itself, but simply
engage the eye region E sufficiently to obtain the tear fluid
sample. Furthermore, the tear fluid sample need not be
self-obtained by the user, but could be sampled from the user by
another individual. In order to stabilize glucose concentrations
across the eye, a user may substantially close his/her eye in order
to perform the self-sampling procedure according to the present
invention. Due to the small sample volume (<0.5 .mu.l) required
by the method and apparatus of the present invention, there is no
need to induce tearing to obtain the tear fluid sample.
[0040] Turning now to FIGS. 2-8, the test apparatus 10 according to
the present invention will now be described. Advantageously, test
apparatus 10 provides the means for obtaining the tear fluid sample
as well as the means for analyzing the glucose concentration of the
tear fluid sample. With reference to FIG. 2, a preferred embodiment
of the test apparatus 10 is shown, wherein test apparatus 10
includes a sample probe, preferably an electrochemical test probe
12 as shown and described herein. Test apparatus 10 preferably
comprises a generally pen-shaped housing 11 for ease of
manipulation by the user while obtaining the tear fluid sample.
[0041] As is known in the art for blood glucose analysis, the test
probe 12 contains chemicals for measuring glucose concentration by
determining the product of an enzymatic reaction, wherein the
selectivity of the enzyme allows for discrimination between glucose
and other substances. In a preferred embodiment, test probe 12 is
constructed similar to that described in International Application
No. PCT/KR02/00703 published on Jul. 10, 2003, which is
incorporated by reference herein. However, in the present
invention, the test probe 12 does not require a blood sample from a
user, but instead accepts a tear fluid sample for analysis of
glucose concentration. Test probe 12 generally includes an input
end for obtaining the tear fluid sample, a probe body for reacting
the tear fluid sample, and an output end for communicating with
test apparatus 10 as described below. Although an electrochemical
test apparatus 10 is shown and described herein, it is understood
that a test apparatus using another analytical technique (e.g.,
fluorescence, absorbance) capable of accurately determining the
concentration of glucose in tear fluid could alternatively be
utilized in accordance with the present invention.
[0042] With reference now to FIGS. 3-8, test probe 12 of the
present invention includes a base plate 14, a cover plate 16, and a
spacer 18 inserted therebetween. Test probe 12 has a sample inlet,
as described below, which is arranged for obtaining the tear fluid
sample via contact with the user's eye, and is preferably free of
sharp edges so as to facilitate contact with the user's eye and lid
margin. In a preferred embodiment, the sample inlet is constructed
to extend outwardly from test probe 12 as shown in FIGS. 3-8 to
further aid in obtaining the tear fluid sample. In the embodiment
depicted in FIGS. 3 and 4, the sample inlet includes a capillary
member 13 that protrudes from test probe 12, wherein opposed halves
of member 13 are molded into each of base and cover plates 14, 16
such that member 13 is formed upon assembly of test probe 12.
Capillary member 13 is placed in contact with the eye region and
the tear fluid sample is drawn up into capillary member 13 by
capillary action. FIGS. 5 and 6 illustrate an alternative capillary
configuration, wherein a capillary member 13' is fully formed in an
auxiliary plate 15 covering cover plate 16. Auxiliary plate 15 and
cover plate 16 are provided with connection apertures 17 and 19,
respectively, which are aligned upon assembly of test probe 12 such
that the tear fluid sample can flow from capillary member 13' to
spacer 18 and base plate 14. In another alternative embodiment
depicted in FIGS. 7 and 8, the sample inlet can include a wicking
membrane 20 for drawing the tear fluid sample into test probe 12.
Wicking membrane 20 can be constructed from materials such as
nitrous cellulose, filter paper, or the like. Of course, it is
understood that test probe 12 may be used without capillary members
13, 13', wicking membrane 20, or the like and contact the eye
region E directly to obtain the tear fluid sample.
[0043] As shown in the exploded view of FIGS. 4, 6, and 8, base and
cover plates 14, 16 of test probe 12 include an electrode system
comprising a working electrode 22 and a reference electrode 24.
Most preferably, the electrode system is of a converse-type in
which working electrode 22 and reference electrode 24 are disposed
on different plates 14, 16 in an opposed, spaced apart
relationship. Such a converse configuration has been shown to allow
for reduced sample volume and measurement time. Of course, other
types of electrode configurations could also be utilized for
carrying out the present invention.
[0044] With continuing reference to FIGS. 3-8, base plate 14
includes working electrode 22 and a first electrode connector 26
provided thereon, and cover plate 16 includes reference electrode
24 and a second electrode connector 28 provided on an underside
thereof. Immobilized on working electrode 22 is an enzyme and an
electron transfer mediator, as described below with reference to
FIG. 9. A view window (not shown) may be provided in cover plate 16
to offer a visual indication of sample uptake into test probe 12.
Base and cover plates 14, 16 are preferably constructed of ceramic,
glass, or polymeric materials, most preferably an organic polymer
of polyester, polyvinyl chloride, or polycarbonate. Working
electrode 22, reference electrode 24, and electrode connectors 26,
28 are constructed using a conductive material, e.g., silver epoxy,
silver/silver chloride, carbon, redox couples, or a modified
conductive carbon paste containing a resin binder. These materials
can be formed into electrodes 22, 24 and electrode connectors 26,
28 by a screen-printing method, an ink jet printing method, a vapor
deposition method followed by etching, an adhesion of a conductive
tape, or the like. Base plate 14 preferably extends rearwardly
beyond cover plate 16 as shown for insertion into test apparatus 10
(see FIGS. 3, 5, and 7).
[0045] Referring again to FIGS. 4, 6, and 8, spacer 18 includes a
channel 32 for introduction of the tear fluid sample into test
probe 12 and a discharge passage 34. Due to capillary action, the
tear fluid sample is introduced into channel 32 via the sample
inlet, such as capillary members 13, 13' or wicking membrane 20,
while any air or excess sample is discharged through discharge
passage 34. The shape of the channel 32 aids in reducing the sample
volume required for glucose concentration measurements. Spacer 18
is preferably constructed by pressing a double-sided adhesive film
made of organic polymer comprising polyester, polyvinyl chloride,
or polycarbonate onto the base, or screen-printing a layer of
adhesive onto base plate 14 with the pattern shown in FIGS. 4, 6,
and 8. Pressing cover plate 16 onto test probe 12, so as to align
and connect electrode connectors 26, 28, completes the circuit and
forms the assembled test probe 12 shown in FIGS. 3, 5, and 7. Test
probe 12 can be constructed to be disposable for one time use, or
alternatively can be constructed to be reusable. In the latter
case, test probe 12 can be coded with an allowed number of uses or
an expiration date after which it should be replaced.
[0046] In operation, the tear fluid sample obtained through contact
of the sample inlet with a user's eye flows through channel 32 of
spacer 18 and into contact with working electrode 22 on base plate
14 of test probe 12. As indicated above, working electrode 22
includes an enzyme and an electron transfer mediator immobilized
thereon for reacting with glucose in the tear sample in order to
determine its glucose concentration. The tear fluid volume required
for the test is less than about 0.5 .mu.l, although it is fully
contemplated that the test probe could be constructed to
accommodate a sample of larger volume. The test time is
approximately 5 seconds.
[0047] For the method and apparatus of the present invention, the
preferred electrochemical reaction schematic is depicted in FIG. 9,
wherein glucose dehydrogenase (GDH) is utilized as the enzyme and a
ruthenium complex is used as the electron transfer mediator (Med).
Given the low level glucose measurements required for tear fluid,
glucose dehydrogenase is preferred over glucose oxidase. The
electron transfer mediator provided for working electrode 22 may
include organometallic compounds (e.g., Fe, Os, Ru containing
derivatives), ferrocene or its derivatives, ferricyanide, quinone
or its derivatives, organic conducting salts, viologen, or other
compounds. However, a ruthenium complex is preferred since both its
oxidized and reduced states in aqueous solution are stable and
reversible, the reduced mediator is non-reactive to oxygen, its
formal potential is low enough to minimize the influence of
interfering materials, the oxidation of the reduced mediator is not
sensitive to pH, and it does not react with electrochemically
interfering materials. As shown, pyrrolo-quinoline-quinone (PQQ) is
preferably used as a coenzyme for glucose dehydrogenase in the
reaction scheme of the present invention.
[0048] As shown in the reaction scheme of FIG. 9, glucose undergoes
an enzymatic reaction wherein glucose is oxidized to gluconic acid
by reducing glucose dehydrogenase (GDH.sub.red). The reduced
glucose dehydrogenase transfers an electron to the electron
transfer mediator (Med.sub.ox) and then returns to an initial state
(GDH.sub.ox). The consequently reduced mediator (Med.sub.red)
becomes reoxidized at the working electrode. The oxidation of the
reduced mediator results in a redox current which is specifically
related to the concentration of the glucose in the tear fluid
sample.
[0049] Referring now to FIGS. 2 and 10, test apparatus 10 includes
a port 36 for removably receiving test probe 12, and contains an
electronic circuit and components (shown schematically in FIG. 10)
for measuring the current resulting from the enzymatic reaction,
similar to readers used for blood glucose analysis. After obtaining
the tear fluid sample, which subsequently flows into contact with
working electrode 22 via capillary action, a power supply 38
applies a voltage to electrode system 21 of test probe 12, which
induces the series of electrochemical reactions for glucose
described above. Electrode system 21 is in communication with an
amplifier 40 such that the resultant current, which is directly
proportional to tear glucose concentration, is amplified, detected
by a sensor such as an ammeter 42 or the like, and subsequently
translated to a value for tear glucose concentration by a processor
44. Processor 44 then correlates the determine tear glucose
concentration with blood glucose concentration, such as by
multiplication with a calibration factor, and the resulting value
is displayed on an LCD display screen 46 (see also FIG. 2) provided
on the test apparatus 10.
[0050] Still referring to FIG. 10, in a preferred embodiment, test
apparatus 10 includes memory 48 for storing each glucose
concentration value, along with the date, time of day, and possibly
other input information for later reference by the user and his/her
physician. Keys 50 (see also FIG. 2) are provided for user input
and data recall purposes, and a clock 52 is provided in
communication with processor 44. A speaker 54 can be provided for
sounding a beep or the like when an adequate test sample volume has
been obtained, advantageously providing the user with an audible
indication that test apparatus 10 can be removed from contact with
their eye region. Speaker 54 may additionally be used to indicate
test completion. Test apparatus 10 can also be configured to upload
glucose concentration data for access by a physician for remote
monitoring purposes. For example, this can be accomplished via a
standard I/O port 56 such as a USB or firewire port or the like, or
by extracting a removable memory card (not shown) and reading the
stored data via a standard format card reader. In addition, test
apparatus 10 can include a temperature controller 58 for regulating
the temperature of the enzymatic reaction.
[0051] In preliminary studies, the self-sampling method and
apparatus of the present invention were evaluated. Each user
learned the self-sampling method in a single session and repeated
it without failure in multiple sessions. Significantly, neither
tear insufficiency (dry eye) nor reflex tearing during
self-sampling was found to have an effect on tear glucose
concentration.
[0052] Glucose solutions in a range of concentrations equivalent to
those reported for tears were tested using the apparatus of the
present invention. The dynamic response of the apparatus to a range
of concentrations of glucose in PBS is shown in FIG. 11, wherein
the currents reach steady-state after approximately 2 seconds. A
calibration curve was constructed for the range of concentrations
of glucose from 0-40 mg/dL (FIG. 12) using solutions of glucose in
a simulated tear matrix (0.01 M phosphate buffer containing 140 mM
NaCl, 5 g/L of bovine serum albumin, pH 7.6). The results showed
high accuracy and precision of glucose measurement, not only in the
normal tear range, but also at levels up to 50% below normal.
Accuracy and precision were tested with concentrations of the
standard glucose solution from 0-28 mg/dL, n=5 (FIG. 13). As shown
in FIG. 14, linearity persists at very low (anticipated
hypoglycemic) glucose concentrations. Precision ranged from 0 to
8.7% CV. As shown in FIGS. 13 and 14, the apparatus according to
the present invention is capable of detecting glucose
concentrations less than about 20 mg/dL with a resolution of at
least 5 mg/dL.
[0053] The method and apparatus of the present invention can be
used to develop a correlational model between tear and blood
concentrations of glucose, allowing the use of tear glucose
readings in place of blood glucose readings to assess circulating
levels of glucose in the body. Such a model may include covariate
adjustment for demographic data such as subject age, gender,
diabetic status, and perhaps other important terms, e.g. weight or
body mass index, medications, and fasting status. Insight into the
kinetics of the correlation between changes in blood and tear
glucose can be assessed dynamically (i.e., as the level of blood
glucose is increasing or decreasing) using the apparatus and method
of the present invention. In addition, diurnal effects on tear
glucose concentration can also be investigated.
[0054] In summary, prior to the development of the method and
apparatus of the present invention, the approach of using tear
glucose to monitor blood glucose has not been practical due to the
low glucose concentration in tears and the sub-microliter sample
volumes available. The method and apparatus described herein are
sensitive to the range of glucose concentrations present in tears,
requires remarkably small sample volume, and uses a test apparatus
that permits direct self-sampling of tears. Importantly, the test
apparatus 10 allows for the sampling and testing of tear fluid
sample with a single device, eliminating any need for intermediate
handling of tears. By overcoming previous limitations, the method
and apparatus of the present invention provide the capability to
measure tear glucose, and therefore monitor blood glucose, in a
simple, clinically practical manner.
[0055] The method and apparatus described herein offer a
replacement to current patient direct measurement of blood glucose
levels which require a finger stick to obtain a blood sample. In
the medical management of diabetes, the glucose level is the
essential information required for decisions on when, how much, and
what type of insulin should be administered. Due to their
completely noninvasive nature, the method and apparatus of the
present invention could significantly improve the quality of
medical care of diabetes through easily repeated testing to prevent
hypoglycemia, the limiting factor in the management of
insulin-dependent diabetes mellitus.
[0056] Although the measurement of glucose concentration in tears
has been described herein, it is understood that the present
invention provides a method and apparatus for obtaining and
evaluating a tear fluid sample that can be used to determine the
concentration of any substance in blood which also manifests a
concentration in tears including, but not limited to, ascorbic
acid, uric acid, albumin, plasma ions, and cholesterol as well as
foreign substances such as drugs.
[0057] While embodiments of the invention have been illustrated and
described, it is not intended that these embodiments illustrate and
describe all possible forms of the invention. Rather, the words
used in the specification are words of description rather than
limitation, and it is understood that various changes may be made
without departing from the spirit and scope of the invention.
* * * * *