To investigate the factors that influence the consistency of international normalized ratio (INR) of prothrombin time.Using the same automated blood coagulation analyzer and INR calibration plasma with two different types of prothrombin time (PT) reagent to assay the INR values of 19 controls (group 1), 12 patients who took long-term anticoagulant orally (group 2), 12 patients who took it at the first time (group 3) and 8 patients who took long-term anticoagulant together with cephalosporin N (group 4).The two kinds of reagents gave quite different INR values in all four groups (P < 0.01). The INR difference in group 3 (0.23 +/- 0.07, n = 12) was larger than that in group 2 (0.51 +/- 0.20, n = 12) (P < 0.01). The difference in group 4 (0.61 +/- 0.21, n = 8) was larger than that in group 2 (P < 0.01).The factors that may influence INR consistency are the source and sensitivity of the reagents, as well as the period of the anti coagulant been taken and the other drugs taken at the same time.