U.S. patent application number 10/620055 was filed with the patent office on 2004-01-22 for electrochemical coagulation assay and device.
Invention is credited to Ohara, Timothy J., Shartle, Robert Justice, Teodorczyk, Maria.
Application Number | 20040011672 10/620055 |
Document ID | / |
Family ID | 24961301 |
Filed Date | 2004-01-22 |
United States Patent
Application |
20040011672 |
Kind Code |
A1 |
Ohara, Timothy J. ; et
al. |
January 22, 2004 |
Electrochemical coagulation assay and device
Abstract
Methods and devices for electrochemically detecting a change in
the viscosity of a fluid are provided. In the subject methods, a
fluid sample is introduced into an electrochemical cell having
oppositely spaced apart working and reference electrodes. An
electric potential is applied to the cell to first achieve a steady
state cell current. A decrease in the steady state cell current is
then detected and related to a change in viscosity of the sample.
In many embodiments, the sample is blood and the change in
viscosity is related to the onset of coagulation in the blood
sample, and often the PT of the blood sample. Also provided are
test strips, kits thereof and meters for use in practicing the
subject methods.
Inventors: |
Ohara, Timothy J.; (San
Ramon, CA) ; Teodorczyk, Maria; (San Jose, CA)
; Shartle, Robert Justice; (Livermore, CA) |
Correspondence
Address: |
BOZICEVIC, FIELD & FRANCIS LLP
200 MIDDLEFIELD RD
SUITE 200
MENLO PARK
CA
94025
US
|
Family ID: |
24961301 |
Appl. No.: |
10/620055 |
Filed: |
July 14, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10620055 |
Jul 14, 2003 |
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09736788 |
Dec 13, 2000 |
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6620310 |
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Current U.S.
Class: |
205/792 ;
204/403.01 |
Current CPC
Class: |
G01N 33/4905 20130101;
B01L 2300/0825 20130101; G01N 33/5438 20130101; A61B 5/150022
20130101; G01N 11/00 20130101; B01L 2400/0406 20130101; A61B
10/0045 20130101; G01N 33/491 20130101; B01L 2300/0672 20130101;
B01L 3/5027 20130101; A61B 5/150358 20130101; G01N 27/307
20130101 |
Class at
Publication: |
205/792 ;
204/403.01 |
International
Class: |
G01N 027/26 |
Claims
What is claimed is:
1. A method for detecting a change in the viscosity of a fluid
sample, said method comprising: (a) introducing a sample into an
electrochemical cell comprising oppositely spaced apart working and
reference electrodes; (b) applying an electric potential to said
reaction cell to produce a steady state current between said
oppositely spaced apart electrodes; (c) detecting a change in said
steady state current; and (d) relating said change in steady state
current to a change in viscosity of said fluid sample.
2. The method according to claim 1, wherein said change in steady
state current is a decrease in steady state current.
3. The method according to claim 1, wherein said change in
viscosity is an increase.
4. The method according to claim 1, wherein said fluid sample is a
physiological sample.
5. The method according to claim 4, wherein said physiological
sample is blood.
6. The method according to claim 5, wherein said method further
comprises relating said change in viscosity to the prothrombin time
(PT) of said blood.
7. The method according to claim 1, wherein said electrochemical
cell comprises a redox couple.
8. A method for detecting the onset of coagulation of a blood
sample, said method comprising: (a) introducing said blood sample
into an electrochemical cell comprising: (i) oppositely spaced
apart working and reference electrodes; and (ii) a reagent mixture
comprising a redox couple; (b) applying an electric potential to
said reaction cell to produce a steady state current between said
oppositely spaced apart electrodes; (c) detecting a change in said
steady state current; and (d) relating said change in steady state
current to the onset of coagulation in said blood sample.
9. The method according to claim 8, wherein said change is a
decrease.
10. The method according to claim 8, wherein said reagent comprises
a coagulation catalyzing agent.
11. The method according to claim 10, wherein said coagulation
catalyzing agent comprises thromboplastin.
12. The method according to claim 10, wherein said method further
comprises relating said onset of coagulation to the prothrombin
time of said blood sample.
13. An electrochemical test strip comprising: an electrochemical
cell comprising: (a) oppositely spaced apart working and reference
electrodes; and (b) a reagent mixture comprising: (i) a redox
couple; and (ii) a coagulation catalyzing agent.
14. The reagent test strip according to claim 13, wherein said
oppositely spaced working and reference electrodes are separated by
a distance ranging from about 50 to 750 .mu.m.
15. The reagent test strip according to claim 14, wherein said
coagulation catalyzing agent comprises thromboplastin.
16. The reagent test strip according to claim 13, wherein said
redox couple comprises a ferricyanide and ferrocyanide.
17. The reagent test strip according to claim 13, wherein said
electrochemical cell has a volume ranging from about 0.1 to 10
.mu.L.
18. A meter for detecting a change in viscosity of a fluid sample,
said meter comprising: (a) means for applying an electric potential
to an electrochemical cell made up of oppositely spaced apart
working and reference electrodes and comprising said fluid sample;
(b) means for measuring cell current between said oppositely space
apart working and reference electrodes; (c) means for detecting a
change in said measured cell current; and (d) means for relating
said change in measured cell current to a change in viscosity of
said fluid sample.
19. The meter according to claim 18, wherein said meter further
comprises a means for relating said change in viscosity to the
prothrombin time of said fluid sample.
20. A kit for use in detecting a coagulation event in a blood
sample, said kit comprising: (a) at least one electrochemical test
strip comprising an electrochemical cell comprising: (i) oppositely
spaced apart working and reference electrodes; and (ii) a reagent
mixture comprising a redox couple and a coagulation catalyzing
agent; and (b) at least one of a calibration means and a means for
obtaining a sample.
Description
FIELD OF THE INVENTION
[0001] The field of this invention is coagulation, and particularly
coagulation testing.
BACKGROUND
[0002] Coagulation is defined as a transformation of a liquid or
sol into a soft, semi-solid or solid mass. Blood naturally
coagulates to form a barrier when trauma or pathologic conditions
cause vessel damage. There are two well-recognized coagulation
pathways: the extrinsic or thromboplastin-controlled and the
intrinsic or prothrombin/fibrinogen-con- trolled coagulation
pathway. Both the extrinsic and intrinsic pathways result in the
production of thrombin, a proteolytic enzyme which catalyzes the
conversion of fibrinogen to fibrin.
[0003] Coagulation tests which measure a blood sample's ability to
form a clot or coagulate have been developed and used to measure
the Prothrombin Time (PT) of a blood sample. Such tests are
commonly referred to as PT tests. PT tests find use in a number of
different applications. For example, PT tests find use in
monitoring patients undergoing anticoagulant therapy. Other
situations where PT tests find use include tests to determine:
acquired platelet function defect; congenital platelet function
defects; congenital protein C or S deficiency; deep intracerebral
hemorrhage; DIC (Disseminated intravascular coagulation); factor II
deficiency; factor V deficiency; factor VII deficiency; factor X
deficiency; hemolytic-uremic syndrome (HUS); hemophilia A;
hemophilia B; hemorrhagic stroke; hepatic encephalopathy;
hepatorenal syndrome; hypertensive intracerebral hemorrhage;
idiopathic thrombocytopenic purpura (ITP); intracerebral
hemorrhage; lobar intracerebral hemorrhage; placenta abruption;
transient ischemic attack (TIA); Wilson's disease; and the like. As
such, PT tests find use in a variety of different applications.
[0004] A number of different PT determination tests and devices
have been developed. Such devices and test protocols include both
optical based devices, such as those described in U.S. Pat. No.
6,084,660; to R. Shartle; and electrochemical based devices, such
as those described in U.S. Pat. Nos. 6,046,051; 6,060,323 and
6,066,504; all to A. Jina. In this latter group of patents a device
is disclosed which is suitable for electrochemical determination of
a change of fluid viscosity in a sample, where the device is
characterized by the presence of side-by-side electrodes. This
configuration requires the use of relatively large volumes of
sample and a measurement protocol that implements a time dependent
deconvolution of the background response; i.e., signal is measured
over time and is then distinguished over background. Thus, the
protocols employed with Jina's devices are more complicated and
perhaps less robust than the protocols used in the present
invention described below.
[0005] While a number of different PT determination tests and
devices have been developed, there continues to be a need for
additional protocols and devices. Of particular interest would be
the development of PT system that provided for rapid and accurate
PT determinations with small sample volumes using inexpensive
device components, such as disposable reagent strips. Of even
greater interest would be the development of an electrochemical
device and protocol which exhibits the above desirable parameters,
is suitable for use with small sample volumes and can provide a
simple-to-interpret signal that converges to a steady-state
value.
[0006] Relevant Literature
[0007] United States Patent of interest include: U.S. Pat. Nos.
6,084,660; 6,066,504; 6,060,323; 6,046,051; 5,942,102; 5,916,522;
5,628,961; 5,554,531; and 5,300,779. Also of interest are WO
97/18465; WO 95/06868; EP 974840 and GB 1 299 363.
SUMMARY OF THE INVENTION
[0008] Methods and devices for electrochemically detecting a change
in the viscosity of a fluid are provided. In the subject methods, a
fluid sample is introduced into an electrochemical cell having
oppositely spaced apart working and reference electrodes. An
electric potential is applied to the cell to first achieve a steady
state cell current. A decrease in the steady state cell current is
then detected and related to a change in viscosity of the sample.
In many embodiments, the sample is blood and the change in
viscosity is related to the onset of coagulation in the blood
sample, and often the PT of the blood sample. Also provided are
test strips, kits thereof and meters for use in practicing the
subject methods.
BRIEF DESCRIPTION OF THE FIGURES
[0009] FIG. 1 provides an exploded view of a reagent test strip
according to the subject invention.
[0010] FIG. 2 shows the time-current plot of a typical data set
where blood is introduced into a strip and the current is monitored
with time.
DESCRIPTION OF THE SPECIFIC EMBODIMENTS
[0011] Methods and devices for electrochemically detecting a change
in the viscosity of a fluid are provided. In the subject methods, a
fluid sample is introduced into an electrochemical cell having
oppositely spaced apart working and reference electrodes. An
electric potential is applied to the cell to first achieve a steady
state cell current. A decrease in the steady state cell current is
then detected and related to a change in viscosity of the sample.
In many embodiments, the sample is blood and the change in
viscosity is related to the onset of coagulation in the blood
sample, and often the PT of the blood sample. Also provided are
test strips, kits thereof and meters for use in practicing the
subject methods.
[0012] Before the subject invention is described further, it is to
be understood that the invention is not limited to the particular
embodiments of the invention described below, as variations of the
particular embodiments may be made and still fall within the scope
of the appended claims. It is also to be understood that the
terminology employed is for the purpose of describing particular
embodiments, and is not intended to be limiting. Instead, the scope
of the present invention will be established by the appended
claims.
[0013] In this specification and the appended claims, singular
references include the plural, unless the context clearly dictates
otherwise. Unless defined otherwise, all technical and scientific
terms used herein have the same meaning as commonly understood to
one of ordinary skill in the art to which this invention
belongs.
[0014] Methods
[0015] As summarized above, the subject invention provides a method
for determining a change in viscosity of a fluid sample. Often the
subject methods provide a means for determining or detecting an
increase in the viscosity of a fluid sample. The subject methods
are sufficiently sensitive to detect an increase in viscosity that
is less than about 1 cps, and often less than about 0.5 cps in
magnitude. As such, the subject methods are sensitive methods for
detecting a change in viscosity of a fluid sample.
[0016] Another feature of the subject methods is that they are
electrochemical methods for determining a change, and often an
increase, in the viscosity of a fluid sample. By electrochemical
methods is meant that the subject methods employ a working and a
reference electrode. Specifically, the subject methods employ a
current produced between a working and reference electrode and
changes therein to determine a change in viscosity of the fluid
sample, as described in greater detail below.
[0017] The first step in the subject methods is to introduce a
quantity of the fluid to be assayed, i.e., a fluid sample, into an
electrochemical cell that includes oppositely spaced apart working
and reference electrodes. The nature of the fluid may vary, so long
as the fluid is a conductor, e.g., an electrolyte. In many
embodiments, the fluid is an aqueous fluid, where of particular
interest are physiological samples. Where the fluid is a
physiological sample, in many embodiments the fluid is whole blood,
or a derivative thereof from which the coagulation/clotting time,
and therefore PT time, can be derived.
[0018] The amount of fluid, e.g., blood, that is introduced into
the electrochemical cell varies, but is generally a small volume.
As such, the volume of fluid introduced into the electrochemical
cell typically ranges from about 0.1 to 10 .mu.L, usually from
about 0.2 to 5.0 .mu.L, and more usually from about 0.3 to 1.6
.mu.L. The sample is introduced into the electrochemical cell using
any convenient protocol, where the sample may be injected into the
electrochemical cell, allowed to wick into the electrochemical
cell, and the like, as may be convenient and depending on the
nature of the device/system in which the subject method is
practiced.
[0019] While the subject methods may be used, in principle, with
any type of electrochemical cell having oppositely spaced apart
working and reference electrodes, in many embodiments the subject
methods employ an electrochemical test strip. The electrochemical
test strips employed in these embodiments of the subject invention
are made up of two opposing metal electrodes separated by a thin
spacer layer, where these components define a reaction area or zone
that makes up the electrochemical cell.
[0020] In certain embodiments of these electrochemical test strips,
the working and reference electrodes are generally configured in
the form of elongated rectangular strips. Typically, the length of
the electrodes ranges from about 1.9 to 4.5 cm, usually from about
2.0 to 2.8 cm. The width of the electrodes ranges from about 0.07
to 0.76 cm, usually from about 0.24 to 0.60 cm. The working and
reference electrodes typically have a thickness ranging from about
10 to 100 nm and usually from about 10 to 20 nm. FIG. 1 provides an
exploded view of an electrochemical test strip according to the
subject invention.
[0021] The working and reference electrodes are further
characterized in that at least the surface of the electrodes that
faces the reaction area of the electrochemical cell in the strip is
a metal, where metals of interest include palladium, gold,
platinum, silver, iridium, carbon (conductive carbon ink), doped
tin oxide, stainless steel and the like. In many embodiments, the
metal is gold or palladium. While in principle the entire electrode
may be made of the metal, each of the electrodes is generally made
up of an inert support material on the surface of which is present
a thin layer of the metal component of the electrode. In these more
common embodiments, the thickness of the inert backing material
typically ranges from about 25 to 500, usually 50 to 400 .mu.m,
e.g., from about 127 to 178 .mu.m, while the thickness of the metal
layer typically ranges from about 10 to 100 nm and usually from
about 10 to 40 nm, e.g. a sputtered metal layer. Any convenient
inert backing material may be employed in the subject electrodes,
where typically the material is a rigid material that is capable of
providing structural support to the electrode and, in turn, the
electrochemical test strip as a whole. Suitable materials that may
be employed as the backing substrate include plastics, e.g. PET,
PETG, polyimide, polycarbonate, polystyrene, silicon, ceramic,
glass, and the like.
[0022] A feature of the electrochemical test strips used in these
embodiments of the subject methods is that the working and
reference electrodes as described above face each other and are
separated by only a short distance, such that the distance between
the working and reference electrodes in the reaction zone or area
of the electrochemical test strip is extremely small. This minimal
spacing of the working and reference electrodes in the subject test
strips is a result of the presence of a thin spacer layer
positioned or sandwiched between the working and reference
electrodes. The thickness of this spacer layer may range from 50 to
750 .mu.m and is often less than or equal to 500 .mu.m, and usually
ranges from about 100 to 175 .mu.m, e.g., 102 to 153 .mu.m. The
spacer layer is cut so as to provide a reaction zone or area with
at least an inlet port into the reaction zone, and generally an
outlet port out of the reaction zone as well. The spacer layer may
have a circular reaction area cut with side inlet and outlet vents
or ports, or other configurations, e.g. square, triangular,
rectangular, irregular shaped reaction areas, etc. The spacer layer
may be fabricated from any convenient material, where
representative suitable materials include PET, PETG, polyimide,
polycarbonate, and the like, where the surfaces of the spacer layer
may be treated so as to be adhesive with respect to their
respective electrodes and thereby maintain the structure of the
electrochemical test strip. Of particular interest is the use of a
die-cut double-sided adhesive strip as the spacer layer.
[0023] The electrochemical test strips used in these embodiments of
the subject invention include a reaction zone or area that is
defined by the working electrode, the reference electrode and the
spacer layer, where these elements are described above.
Specifically, the working and reference electrodes define the top
and bottom of the reaction area, while the spacer layer defines the
walls of the reaction area. The volume of the reaction area
typically ranges from about 0.1 to 10 .mu.L, usually from about 0.2
to 5.0 .mu.L, and more usually from about 0.3 to 1.6 .mu.L. As
mentioned above, the reaction area generally includes at least an
inlet port, and in many embodiments also includes an outlet port.
The cross-sectional area of the inlet and outlet ports may vary as
long as it is sufficiently large to provide an effective entrance
or exit of fluid from the reaction area, but generally ranges from
about 9.times.10.sup.-4 to 5.times.10.sup.-3 cm.sup.2, usually from
about 1.3.times.10.sup.-3 to 2.5.times.10.sup.-3 cm.sup.2.
[0024] In many embodiments, a reagent system is present in the
reaction area, where the reagent system interacts with components
in the fluid sample during the assay. For example, in embodiments
where the subject methods are used to detect a coagulation event,
e.g., to measure PT of a sample, the reaction area or zone includes
a reagent system that at least includes a redox couple, and often
also includes a coagulation catalyzing agent.
[0025] The redox couple of the reagent composition, when present,
is made up of one or more redox couple agents. A variety of
different redox couple agents are known in the art and include:
ferricyanide, phenazine ethosulphate, phenazine methosulfate,
pheylenediamine, 1-methoxy-phenazine methosulfate,
2,6-dimethyl-1,4-benzoquinone, 2,5-dichloro-1,4-benzoquinone,
ferrocene derivatives, osmium bipyridyl complexes, ruthenium
complexes, and the like. In many embodiments, redox couples of
particular interest are ferricyanide, and the like.
[0026] In many embodiments, the reagent composition also includes a
coagulation catalyzing agent. By coagulation catalyzing agent is
meant one or more components or reactants that participate or
interact with components present in the fluid sample, e.g., whole
blood, to initiate the clotting process in the blood sample. For PT
assays, the coagulation catalyzing agent generally comprises
thromboplastin, which thromboplastin may be purified from a
naturally occurring source, e.g., an aqueous extract of acetone
dried brain tissue, or synthetic recombinant thromboplastin (r-DNA
thromboplastin), which generally includes purified recombinant
tissue factor protein and a purified artificial lipid component. A
representative coagulation catalyzing agent is thromboplastin-XS
with calcium sold under the trade name INNOVIN.RTM. by Dade
International, Miami Fla.
[0027] Other reagents that may be present in the reaction area
include buffering agents, e.g. citraconate, citrate, malic, maleic,
phosphate, "Good" buffers and the like. Yet other agents that may
be present include: divalent cations such as calcium chloride, and
magnesium chloride; surfactants such as Triton, Macol, Tetronic,
Silwet, Zonyl, and Pluronic; stabilizing agents such as albumin,
sucrose, trehalose, mannitol, and lactose.
[0028] The reagent system, when present, is generally present in
dry form. The amounts of the various components may vary, where the
amount of the oxidized redox couple component typically ranges from
about 5 to 1000 mM, usually from about 90 to 900 mM; the reduced
redox couple component typically ranges from about 1 to 20 mM,
usually from about 5 to 15 mM; the amount of buffer typically
ranges from about 0 to 300 mM, usually from about 50 to 100 mM; and
the amount of coagulation catalyzing agent component typically
ranges from about 0.005 to 50 mg/cm.sup.2, usually from about 0.05
to 5 mg/cm.sup.2. The overall mass of dry reagent present in the
reaction area or zone in these embodiments generally ranges from
about 4 to 700 ng/cm.sup.2, usually from about 8 to 350
ng/cm.sup.2.
[0029] A representative test strip for use in the subject methods
is depicted in exploded view in FIG. 1.
[0030] Following sample introduction into the electrochemical cell,
a constant electric potential is applied to the cell in a manner
sufficient to produce a steady state current between the working
and reference electrodes of the cell. More specifically, a constant
electric potential is applied between the working and the reference
electrodes in a manner that produces a steady state current between
the two electrodes. The magnitude of the applied electric potential
generally ranges from about 0 to -0.6 V, usually from about -0.2 to
-0.4 V. In many embodiments where the electrochemical cell includes
a redox couple, as described above, application of the constant
electrical potential as described above results in the production
of a steady state current described by the following formula:
i.sub.ss=n2FADCo/L;
[0031] where:
[0032] n is equal to the number of electrons transferred;
[0033] F is Faraday's constant, i.e.,
9.6485.times.10.sup.4C/mol;
[0034] A is the area of the working electrode;
[0035] D is the diffusion coefficient of the cell, where this
coefficient may be determined from Fick's first law, i.e.
J(x,t)=-D.sup.dCo(x,t)/dx where j is flux, x is the position from
the electrode, and t is time;
[0036] Co is the redox couple concentration, e.g., the ferrocyanide
concentration; and
[0037] L is distance between the electrodes, e.g., the spacer
thickness.
[0038] The overall time period required to obtain the requisite
steady state current, as described above, is relatively short in
certain embodiments. In such embodiments, the total amount of time
required to obtain the steady state current, i.e., the period from
sample entry to the cell to establishment of the steady state
current, is less than about 15 seconds, usually less than about 10
seconds; and often ranges from about 4 to 15 seconds.
[0039] FIG. 2 shows the time-current plot of a typical data set
where blood is introduced into a strip and the current is monitored
with time.
[0040] The next step in the subject methods is to detect a change
in the steady state current and relate this change to a change in
viscosity of the sample. In many embodiments, the change that is
detected is a decrease in the steady state current. The magnitude
of the decrease in the steady state current that is detected in
this step is at least about 2%, and usually at least about 10%,
where the magnitude of the decrease in many embodiments ranges from
about 2 to 90%. In other embodiments, of interest is the rate of
change between two steady state values, one before and one after
the coagulation event, and the relation of this change in rate to
the presence of the coagulation event.
[0041] The detection of the above described decrease in steady
state current is then related to an increase in viscosity of the
fluid sample in the electrochemical cell. Relatively small
increases in viscosity result in a detectable decrease in the
steady state current and thus can be detected by the subject
methods, where the magnitude of the increase in viscosity may be as
small as 0.5 cps or smaller in certain embodiments.
[0042] In many embodiments where the sample present in the
electrochemical cell is whole blood and the reagent composition
includes a coagulation catalyzing agent, the increase in viscosity
is then related to the onset of coagulation in the blood sample,
i.e., the occurrence of a coagulation event or blood clotting in
the blood sample.
[0043] In certain embodiments, the increase in viscosity and
concomitant detection of the onset of coagulation in the blood
sample being assayed is employed to determine the PT of the blood
sample. In these embodiments, the period extending from the initial
sample introduction into the reaction area or zone and/or the
establishment of a steady state current and increase in
viscosity/onset of coagulation is determined and the PT of the
blood sample is derived from this time period. The time at which
sample enters the electrochemical cell may be detected using any
convenient protocol, where particular protocols employed may
depend, at least in part, on the nature of the meter device
employed with the electrochemical cell. In certain embodiments, the
time that sample is introduced directly into the reaction cell can
be manually recorded. Alternatively, the meter may automatically
detect sample introduction into the electrochemical cell, e.g., by
detecting an initial decrease in the voltage required to achieve a
constant current between the working and reference electrodes of
the cell. (See U.S. application Ser. No. 9/333,793, filed Jun. 15,
1999, incorporated herein by reference.) Other protocols for sample
detection in the cell may also be employed.
[0044] The above computational steps of the subject method, e.g.,
relation of the time period from sample introduction to onset of
coagulation to the PT of the blood sample, may be accomplished
manually or through the use of an automated computing means, where
in many embodiments the use of an automated computing means, such
as is described in connection with the subject devices discussed
below, is of interest.
[0045] The above described protocol may be carried out at room
temperature or at an elevated temperature. Typically, the above
protocol is carried out at a temperature ranging from about 20 to
40.degree. C., e.g., about 37.degree. C.
[0046] The above described methods find use in any application
where the determination of a viscosity change in a fluid sample is
desirable. As such, the subject methods suited for use in the
determination of PT of a blood sample, and as such find use in any
application where the determination of PT is desired, e.g., those
applications described in the Background Section, supra.
[0047] Devices
[0048] Also provided by the subject invention are meters for use in
practicing the subject invention. The subject meters are typically
meters for measuring a change in viscosity of fluid sample, and are
meters for measuring the PT of a blood sample in many embodiments.
The subject meters typically include: (a) a means for applying an
electric potential to an electrochemical cell into which the sample
has been introduced; (b) a means for measuring cell current in the
cell, including a steady state current in the cell; (c) a means for
detecting a change in the steady state current in the cell, e.g., a
decrease in the steady state current of the cell; and (d) a means
for relating the change in steady state current to a change in
viscosity of the cell, e.g., a means for relating a decrease in
steady state current in the cell to an increase in viscosity of
fluid in the cell.
[0049] The means for applying an electric potential to the
electrochemical cell, means for measuring a steady state current in
the cell and means for detecting a change in the steady state
current in the cell may be any convenient means, where
representative means are described in WO 97/18465 and U.S. Pat. No.
5,942,102; the disclosures of which are herein incorporated by
reference. See also U.S. Pat. Nos. 6,066,504; 6,060,323; 6,046,051;
the disclosures of which are herein incorporated by reference. The
means for relating the change in steady state current to a change
in viscosity is typically a computing means present in the meter
which is capable of relating the measured change in steady state
current to a change in viscosity of the fluid sample. In many
embodiments, this means is further a means for relating the change
in current/viscosity to the onset of coagulation, and is often a
means for determining the PT of a blood sample. See e.g., U.S. Pat.
No. 6,066,504; the disclosure of which is herein incorporated by
reference.
[0050] Kits
[0051] Also provided are kits for use in practicing the subject
methods. The kits of the subject invention at least include an
electrochemical reagent test strip, as described above. The subject
kits may further include a means for obtaining a physiological
sample. For example, where the physiological sample is blood, the
subject kits may further include a means for obtaining a blood
sample, such as a lance for sticking a finger, a lance actuation
means, and the like. In addition, the subject kits may include a
calibration means for calibrating the instrument, e.g., a control
solution or standard, e.g., a coagulation control solution that has
a known PT time. In certain embodiments, the kits also include an
automated instrument, as described above, for detecting the amount
of product produced on the strip following sample application and
relating the detected product to the amount of analyte in the
sample. Finally, the kits include instructions for using the
subject kit components in the determination of an analyte
concentration in a physiological sample. These instructions may be
present on one or more of the packaging, a label insert, containers
present in the kits, and the like.
[0052] The following examples are offered by way of illustration
and not by way of limitation.
Experimental
[0053] I. Electrochemical Test Strip Preparation
[0054] An electrochemical test strip consisting of two metallized
electrodes oriented in a sandwich configuration was prepared as
follows. The top layer of the test strip was a gold sputtered Mylar
strip. The middle layer was a double-sided adhesive with a punched
hole that defined the reaction zone or area. The punched hole was a
circle with two juxtaposed rectangular inlet and outlet channels.
The bottom layer of the test strip was sputtered palladium on
Mylar. A reagent of citraconate buffer, ferricyanide, ferrocyanide
and relipidated recombinant tissue factor was ink jetted on the
palladium sputtered surface. The amount of reagent ink jetted onto
the palladium sputtered surface was 597 ng/cm.sup.2. As such, the
amount of citraconate buffer was 120 ng/cm.sup.2 ferricyanide was
460 ng/cm.sup.2, the amount of ferrocyanide was 8 ng/cm.sup.2 and
the amount of recombinant tissue factor was 9 ng/cm.sup.2. An
exploded view of the test strip is shown in FIG. 1.
[0055] II. Detection of PT
[0056] The above described strip is employed to determine the PT of
a blood sample as follows. A 1.5 .mu.l blood sample is introduced
into the reaction area or zone of the test strip and the sample
introduction time is recorded. A constant potential of -0.3 V is
applied between the working and reference electrodes, and the
resultant current between the two electrodes is monitored. The
appearance of a steady state current is first detected, followed by
a decrease in the steady state current. The time period from the
initial sample introduction to the decrease in steady state current
is determined and then related to the PT of the blood sample. FIG.
2 shows the time-current plot of the data set where blood is
introduced into a strip and the current is monitored with time.
[0057] The above results and discussion demonstrate that subject
invention provides a simple and powerful tool to determine the PT
of a blood sample. Advantages of the subject methods over
non-electrochemical based coagulation detection methods include use
of low cost materials and the opportunity to use wide variety of
controls, including plasma based controls. Additional advantages of
the subject invention include the ability to employ small sample
volumes and the fact that the electrochemical measurements made by
the subject methods provide a simple-to-interpret signal that
converges to a steady-state value. Yet another advantage is the
ability to use low cost electrochemical based meters, which provide
for significant cost savings. As such, the subject invention
represents a significant contribution to the art.
[0058] All publications and patents cited in this specification are
herein incorporated by reference as if each individual publication
or patent were specifically and individually indicated to be
incorporated by reference. The citation of any publication is for
its disclosure prior to the filing date and should not be construed
as an admission that the present invention is not entitled to
antedate such publication by virtue of prior invention.
[0059] Although the foregoing invention has been described in some
detail by way of illustration and example for purposes of clarity
of understanding, it is readily apparent to those of ordinary skill
in the art in light of the teachings of this invention that certain
changes and modifications may be made thereto without departing
from the spirit or scope of the appended claims.
* * * * *