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.
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