How It Works
The Verification Principle
The INR formula is deterministic: given a PT value, an ISI, and a Normal Patient Mean PT, there is exactly one correct INR result. This means the expected INR for any specimen can be independently recalculated using the correct parameters, then compared to what the analyzer actually reported. If they match (within rounding tolerance), the analyzer is correctly programmed. If they do not match, the programmed parameters are wrong.
Calculated INR
Calculated INR = (Analyzer PT / Normal Patient Mean PT)^ISI
Computed using the PT value as reported by the analyzer, the Normal Patient Mean PT established for the current reagent lot (from the Geometric Mean & VRI study), and the ISI from the reagent lot's package insert. This is the INR the analyzer should have reported if its parameters are correctly programmed.