The INR standardises prothrombin time results so that warfarin monitoring is comparable between laboratories that use different reagents. This tool computes the INR from the patient and mean normal prothrombin times and the reagent’s International Sensitivity Index.
How it works
The calculation has two steps:
PT ratio = patient PT (s) / mean normal PT (s)
INR = PT ratio ^ ISIThe mean normal prothrombin time (MNPT) is the geometric mean of prothrombin times from at least 20 healthy donors for the specific reagent and analyser. The ISI, printed on the reagent insert, calibrates that thromboplastin against the international reference preparation, so raising the ratio to the ISI removes the between-reagent variation.
Why the ISI exists: the between-laboratory problem
Before INR standardisation, a patient anticoagulated on warfarin could travel to another country and receive a wildly different reported PT result — not because their coagulation had changed, but because the laboratory used a different thromboplastin reagent with a different sensitivity. Some reagents are highly sensitive to warfarin-induced factor reduction; others are much less so. Two patients with the same actual degree of anticoagulation could show PT ratios of 2.0 and 3.5 depending solely on which reagent was used.
The World Health Organization addressed this by establishing an international reference preparation and requiring each commercial thromboplastin to be calibrated against it. The result of that calibration is the ISI: an ISI of 1.0 means the reagent behaves identically to the reference standard; an ISI of 1.5 means it is less sensitive and the raw ratio must be corrected upward to express the equivalent result.
Worked examples
Example 1 — sensitive modern recombinant thromboplastin:
- Patient PT: 28 s, MNPT: 12 s, ISI: 1.05
- PT ratio = 28 / 12 = 2.333
- INR = 2.333 ^ 1.05 ≈ 2.45
Example 2 — less sensitive reagent from the same patient blood:
- Patient PT: 24 s, MNPT: 12 s, ISI: 1.20
- PT ratio = 24 / 12 = 2.00
- INR = 2.00 ^ 1.20 ≈ 2.30
The PT ratio differs (2.333 vs 2.00) because the two reagents give different raw seconds for the same degree of anticoagulation, but after ISI correction the INR values are similar (~2.45 vs ~2.30), which is the whole point of standardisation.
What to watch for
Use the ISI for the current reagent lot. ISI can vary between lots of the same product. Most manufacturers supply a lot-specific ISI on the insert; some laboratories verify this experimentally. Using a stale or wrong ISI is a systematic source of error.
MNPT must be instrument- and reagent-specific. The MNPT from one analyser cannot be used with a different analyser or reagent. Laboratories establish their own MNPT from a local healthy donor panel, or the manufacturer supplies it for the specific reagent–instrument combination.
INR is invalid for direct oral anticoagulants. Apixaban, rivaroxaban, and dabigatran affect the PT to varying degrees, but the INR was not designed and is not validated for monitoring them. Do not interpret an elevated INR in a patient on a DOAC as equivalent to over-anticoagulation with warfarin.
Example and notes
A patient prothrombin time of 24 seconds with a mean normal prothrombin time of 12 seconds gives a PT ratio of 2.0. With a sensitive modern reagent of ISI 1.0, the INR is 2.0; with a less sensitive reagent of ISI 1.2, the same ratio gives an INR of about 2.30. This is exactly why the ISI matters — the raw ratio is identical but the standardised result differs. Always use the ISI for the exact reagent lot in use, and remember the INR is valid only for vitamin K antagonists, not direct oral anticoagulants.