Abbott i-STAT 1 System Manual - page 493
PROTHROMBIN TIME /
(PT/INR)
Rev. Date: 01-Mar-12
Art: 715236-00N
The i-STAT
®
PT/INR test is a whole blood determination of the prothrombin time used for monitoring oral
anticoagulant (Coumadin or warfarin) therapy. The test determines the time required for complete activation
of the extrinsic pathway of the coagulation cascade when initiated (activated) with a thromboplastin.
In a prothrombin time test, coagulation is initiated by mixing the sample with tissue thromboplastin. In
traditional prothrombin time tests, complete activation is indicated when activated thrombin converts
fibrinogen to fibrin and extensive or localized clots are detected mechanically or optically. The i-STAT PT/INR
test is similar except that the endpoint is indicated by the conversion of a thrombin substrate other than
fibrinogen. An electrochemical sensor is used to detect this conversion.
The added thrombin substrate is H-D-phenylalanyl-pipecolyl-arginine-p-amino-p methoxydiphenylamine, which
has the structure:
Phenylalanine - Pipecolic acid - Arginine -- NH - C
6
H
4
- NH - C
6
H
4
- OCH
3
Thrombin cleaves the amide bond at the carboxy terminus of the arginine residue (denoted by the two
dashes) because the bond structurally resembles the thrombin-cleaved amide linkage in fibrinogen. The
product of the thrombin-substrate reaction is the electrochemically inert tripeptide Phenylalanyl - Pipecolyl -
Arginine and the electroactive compound NH
3
+
- C
6
H
4
- NH - C
6
H
4
- OCH
3
. A formation of the electroactive
compound is detected amperometrically and the time of detection is measured.
The PT/INR test result is reported as an International Normalized Ratio (INR) and, optionally, in seconds. The
INR is the recommended method of result reporting for monitoring of oral anticoagulant therapy
1
. A Mean
Normal i-STAT prothrombin time (sec) and an ISI are determined following the WHO recommendations at a
CAP-accredited facility. INR results are calculated using the following equation:
INR =
Patient i-STAT prothrombin time (sec)
ISI
Mean Normal i-STAT prothrombin time (sec)
The optionally displayed units of seconds reflect traditional plasma PT times. The reported time is derived
from the PT/INR result and the equation below using an ISI of 1.05 and a typical Mean Normal Plasma PT
time of 12.0 seconds.
INR =
Patient Plasma prothrombin time (sec)
ISI
Mean Normal Plasma prothrombin time (sec)
If results appear inconsistent with the clinical assessment, the patient sample should be recollected and
retested using another cartridge.
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