The international normalized ratio (INR) on two widely used point-of-care test (POCT) prothrombin time (PT) monitors (CoaguChek Mini and TAS PT-NC) differed considerably and also differed from the 'true' INR obtained on the same samples using a manual PT and the same species thromboplastin international reference preparation. Agreement between the displayed INR and difference from 'true' INR has been reassessed following an independent international sensitivity index (ISI) calibration of the two systems. The displayed INRs taken at seven centres were compared with 'true' INRs from the same blood donations and INRs based on the resulting ISI. The overall difference between the displayed INRs on the two monitor systems was reduced from 21.0% to 3.5%. The overall difference in mean INR of system A from the 'true' INR was reduced from 19.0% to 9.5% and of system B from 6.8% to 0.3%, but individual centre's results still showed considerable mean INR variability. Differences between overall displayed INR with the two monitor systems have been reduced by an independent multicentre calibration, and agreement with 'true' INR on the same blood samples improved. However, marked variability in mean INR at individual centres remained after ISI correction, which demonstrates the need for external quality control of individual POCT whole-blood PT monitors.