ITC ProTime Microcoagulation System - Manual (Page 31)

31
PERFORMANCE CHARACTERISTICS
Reporting ProTime Results
The ProTime Microcoagulation System reports a result as International Normalized Ratio (INR) and in PT
seconds. The ProTime system calculates the INR directly from whole blood clotting time based on a
conversion equation that was established in clinical trials. The result in plasma equivalent seconds is then
calculated from the INR result.
Historically, laboratories have reported prothrombin time results in seconds, specifically the time it takes
for the plasma sample to clot following the addition of the reagents. Reporting the results in seconds is
problematic since the clotting time depends on the sensitivity of the reagent used. Results cannot be
compared from one laboratory to another. The INR system was introduced by the World Health
Organization (WHO) to standardize PT reporting such that patient results could be uniform across different
laboratories. INR is calculated using the formula:
INR = (PT/PT
n
) ISI
where PT is the time for the patient plasma sample to clot, PT
n
is the mean clotting time of a non-
anticoagulated population, and ISI is the international sensitivity index that is assigned by the manufacturer
of the reagent according to a standard protocol developed by the WHO. The ISI value ranges from 1.0 to
approximately 3.0. A low ISI indicates that the reagent has the highest sensitivity to vitamin K-dependent
clotting factors. Typically, the variable PT
n
is generated by the laboratory with each new reagent lot by
running 20 samples and calculating the mean.
The ProTime system enhances clinical utility for the physician and patient by providing results in both
formats, so that anticoagulant dosing can be managed using a familiar format. Since results reported in PT
seconds depend on the sensitivity (ISI) of the reagent employed, the physician has the option of changing
the ISI value in ProTime so that the ProTime results reported in PT seconds closely match the results
reported by the hospital laboratory. To change the ISI used in the calculation, the physician simply accesses
the programming screens and enters the ISI of the historical laboratory reagent. (Please see the For
Professionals Only – Program Mode section).
The default parameters used to calculate PT seconds are ISI = 1.0, which is the sensitivity of the
thromboplastin reagent used in the ProTime cuvette, and PT
n
= 13.1 seconds, which was established in
clinical trials. The PT
n
value used in the calculation is automatically adjusted when the reporting ISI is
changed.
Table 1. Options for programming ISI and associated PT
n
ISI 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6
PT
n
13.1 12.9 12.8 12.6 12.5 12.3 12.2 12.0 11.9
Table 2. Relationship of INR to PT seconds with varied ISI
PT seconds vs. ISI
INR
ISI = 1.0 ISI = 1.6 ISI = 2.0 ISI = 2.4
1.0 13.1 12.6 12.3 12.0
1.5 19.6 16.2 15.1 14.2
2.0 26.1 19.4 17.4 16.0
2.5 32.6 22.3 19.5 17.6
3.0 39.2 25.0 21.3 19.0
3.5 45.7 27.6 23.0 20.2
4.0 52.2 30.0 24.6 21.4
4.5 58.7 32.3 26.1 22.5