Warfarin Dose Adjustment Calculator

This calculator estimates a weekly warfarin dose adjustment from a current INR lab result, using a simplified empiric percentage-based approach in the style of common hospital pharmacy dosing nomograms: a fixed 15% increase when INR is under 1.5, a fixed 10% increase for INR 1.5 to just under 2.0 when the target is at or above 2.0 (or, if the target itself is also under 2.0, the same proportional fine-tune described next, rather than the fixed step), a proportional fine-tune toward the target INR for values from 2.0 to 3.0, and fixed decreases of 10%, 15%, or 20% as INR climbs through 3.0-4.0, 4.0-5.0, and above 5.0. Every proportional fine-tune step is capped at +/-20%, since no real anticoagulation nomogram adjusts a maintenance dose by more than roughly that much in one step and warfarin's dose-INR response is supra-linear -- an uncapped ratio toward a distant target (for example, current INR 2.0 against a target of 5.0) would otherwise imply an unrealistic dose escalation far beyond what any real protocol recommends. The current weekly dose is a pure multiplier -- it never changes which percentage band applies, it only scales the resulting suggested dose, so doubling the current dose exactly doubles the suggested dose for the same INR values.

This calculator does NOT account for the clinical indication (atrial fibrillation, mechanical heart valve, and VTE treatment carry different target INR ranges and risk tolerances), genetic factors affecting warfarin metabolism (CYP2C9, VKORC1), diet or drug interactions that alter INR independently of dose, or bleeding risk scores. Critically, it also does not represent real supratherapeutic-INR protocols: a clinically significant elevated INR often calls for holding one or more doses, or administering vitamin K, not simply a percentage dose reduction on the same schedule -- this tool has no concept of a "hold dose" or vitamin K recommendation. Because a small INR measurement error can shift the suggested dose meaningfully, and because true dosing decisions weigh bleeding history, recent INR trend, and upcoming procedures, this is a starting-point estimate for clinician review, not a substitute for individualized anticoagulation management.