The objective was to evaluate the prothrombin time in patients treated with warfarin, treated in the Escalabs Laboratory in Trujillo, September 2016-January 2017. Method: descriptive and cross-sectional, the sample was 198 patients of both sexes, criteria were considered for the use of information, the prothrombin time was evaluated and standardized using the international normalized coefficient (INR). The results were: 64.14% of the population are women and 35.86% are men. 54.70% of women and 49.30% of men presented optimal values of the INR with a range of 2-3; but 24.79% of women and 19.72% have an INR greater than 3, so it must be monitored to avoid possible bleeding. Warfarin inhibits epoxide reductase and prevents hepatic gammacarboxylation, that is, acts as "antivitamin K" and prevents the formation of blood clots. The prothrombin time test measures the time it takes for a clot to...