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INR Calculation
The HEMOCHRON Jr. Prothrombin Time (PT, J201) and Citrate Prothrombin
Time (PT, J201C) tests are microcoagulation assays intended for use
in performing quantitative, one stage prothrombin times. Both assays
require whole blood samples, either fresh (J201) or anticoagulated
with sodium citrate (J201C). The thromboplastin reagents in these
cuvettes are highly sensitive, low ISI (approximately 1.0) reagents
providing optimal sensitivity for Vitamin K dependent clotting factors.
The
HEMOCHRON Jr series instruments calculate the INR of the sample directly
from the whole blood clotting time based on regression analyses performed
across multiple centers during the assay development. The plasma
equivalent PT is calculated from the INR based upon an ISI of 1.0.
This differs significantly from the routine laboratory calculation
of INR which first requires calculating the ratio of the patient’s
PT and a local mean normal PT, then raising this ratio to the power
of the ISI.
The decision to directly calculate the INR from the whole
blood clotting time was made to minimize the imprecision introduced
by employing several extra mathematical steps. Since whole blood
clotting times are, by their nature, longer than plasma clotting
times, use of the traditional equation in a whole blood system would
first require the conversion of the whole blood clotting time to
a plasma equivalent value. This value could then be used for the
standard INR equation. Mathematically, imprecision is introduced
into a system with each calculation performed, therefore, the more
direct conversion of whole blood clotting time to INR is preferred.
Local
adjustment of the PT mean normal is therefore unnecessary when using
any HEMOCHRON Jr series instrument. The mean normal PT programmed
into the system is only used to calculate the plasma equivalent clotting
time from the INR, not vice versa.
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