How this calculator works
INR standardizes the prothrombin time ratio using the international sensitivity index for the reagent and instrument system. The denominator is the laboratory mean normal prothrombin time, not an arbitrary control time or a universal 12-second constant. This page performs the equation and inverse algebra only. ISI must come from laboratory calibration; inferring it here is an algebra exercise, not a substitute for calibration. The result is an estimate for information only and is not medical advice.
Formula
INR = (patient PT / mean normal PT)^ISI. Patient PT = mean normal PT x INR^(1/ISI). ISI = ln(INR) / ln(patient PT / mean normal PT).
Worked example
Patient PT 24 s, mean normal PT 12 s and ISI 1 give INR 2. Patient PT 18 s, mean normal PT 12 s and ISI 2 give INR 2.25.
Advertisement
Frequently asked questions
Is a single healthy control the mean normal PT?
No. Use the mean normal PT established for the laboratory test system.
Can this choose an anticoagulant dose?
No. It does not give a dose, target INR or treatment adjustment.
Why cannot equal PT values recover ISI?
Their ratio is one, whose power is one for every ISI; there is no unique solution.