Within-subject and between-subject biological variation of prothrombin time and activated partial thromboplastin time.

The within-subject and between-subject biological variation of prothrombin time (PT) and activated partial thromboplastin time (APTT) have been studied in a group of 19 women and 20 men over a period of 9 months. All measurements were performed with a mechanical coagulometer. The between-subject coefficients of variation were 6.8% for PT and 8.9% for APTT. The medians and the 90th percentiles of within-subject coefficients of variation were 1.7% and 5.9%, respectively, for PT, and 0.0% and 6.1% for APTT. These data were used to calculate the critical difference for significant change detection and the desirable between-day imprecision.