Warfarin dosing is adjusted based on how a patient's INR compares to their target range, using standardized percentage-adjustment nomograms developed by anticoagulation clinics. This calculator applies one such nomogram as a reference tool — it helps clinicians, pharmacists, and patients see what a standard protocol would suggest, while making clear that the actual decision always belongs to the prescriber managing the patient's care.

How the nomogram works

The calculator classifies the current INR into one of seven bands relative to the selected target range: markedly subtherapeutic, mildly subtherapeutic, in range, mildly supratherapeutic, moderately supratherapeutic, markedly supratherapeutic, and critical. Each band (other than in-range and critical) carries a published percentage range for adjusting the total weekly warfarin dose, plus a recheck interval that gets shorter as the INR moves further from target.

These bands and percentages implement a standard institutional anticoagulation-clinic nomogram for a goal INR of 2.0-3.0 (GlobalRPH). For the High target range (2.5-3.5, used for some indications such as mechanical heart valves), every boundary is shifted by the same 0.5 offset applied to the target range itself, since no separately published percentage nomogram exists specifically for that range.

Inputs and what they mean

Current INR is the patient's most recent lab result. Target INR range should match what the prescriber has set for this patient's specific indication — most patients target 2.0-3.0, while some (for example those with mechanical heart valves) target a higher range such as 2.5-3.5. Current total weekly dose is the sum of all warfarin doses the patient takes across a full week, in milligrams — not a single daily dose, since warfarin doses are frequently uneven across the week (e.g. a different dose on one day).

The output — a suggested percentage adjustment, a reference new weekly dose range, and a recheck interval — is a starting point for a clinical conversation, not a final answer. It does not account for bleeding risk factors, drug or dietary interactions, missed doses, or the reason a patient's INR moved outside range in the first place, all of which a prescriber will weigh before making an actual change.

Limits and edge cases

This calculator never instructs a dose change — it shows a reference suggestion from a standard nomogram, and every result explicitly asks the user to discuss it with the prescriber managing the patient's anticoagulation before anything changes. Warfarin has a narrow therapeutic index: too little dose raises clotting risk, too much raises bleeding risk, and the correct adjustment for a specific patient depends on factors this tool cannot see, including active bleeding, recent illness, new medications, and dietary vitamin K intake.

For an INR at or above approximately 5.0, the calculator flags a critical value requiring urgent clinician contact rather than offering a percentage adjustment — decisions at this level, including whether a reversal step such as vitamin K is appropriate, require direct clinical evaluation. Similarly, the 4.0-4.9 band's suggested adjustment assumes no active bleeding; any bleeding symptoms at any INR level require immediate medical attention regardless of this tool's output.