Not medically reviewed
Coagulation
International normalised ratio (INR)
Not medically reviewed · Content last updated · 1 min read ·
This is the date the draft was written, not a medical review date.
The INR is the patient prothrombin time divided by the mean normal prothrombin time, raised to the power of the reagent ISI. It standardises monitoring of vitamin K antagonists across laboratories.
Reference interval
| Conventional (ratio) | SI (ratio) | |
|---|---|---|
| Adults | 0.8–1.2 ratio | 0.8–1.2 ratio |
Specimen: Citrated plasma · Conventional × 1 = SI
Causes of high values
Vitamin K antagonist therapy (intended effect or overdose), liver disease, vitamin K deficiency, DIC, and some antibiotics or drug interactions potentiating warfarin.
Causes of low values
Values below 0.8 are rarely significant; in patients on warfarin a sub-therapeutic INR indicates under-anticoagulation (missed doses, dietary vitamin K, interacting drugs).
Pre-analytical and method notes
The reference interval is 0.8-1.2 in people not anticoagulated; therapeutic targets differ by indication (commonly 2.0-3.0, and 2.5-3.5 for some mechanical valves) and should follow local guidelines. The INR was validated for vitamin K antagonists and is less reliable in liver disease, DOAC therapy and lupus anticoagulant.
References
- Rifai N, Horvath AR, Wittwer CT. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Elsevier, 6th edition (2018) — Tier 3 (textbook)
- Loscalzo J, et al.. Harrison's Principles of Internal Medicine. McGraw-Hill, 21st edition (2022) — Tier 3 (textbook)
- McPherson RA, Pincus MR. Henry's Clinical Diagnosis and Management by Laboratory Methods. Elsevier — Tier 3 (textbook)
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