To evaluate the necessity of duplicate testing for prothrombin times (PTs) and activated partial thromboplastin times (APTTs), the range of differences between duplicate sample results was analyzed on a widely used automated photo-optical coagulation instrument. Specimens with coagulation test times ranging from normal to threefold above the reference range were included. Of 1,610 PTs and 1,023 APTTs, approximately 95% of duplicates differed by 0.2 s or less and 2.0 s or less, respectively. Approximately 99% of PTs and APTTs differed by 0.4 s or less and 4.0 s or less, respectively; there were three PT and 16 APTT specimens whose duplicates differed by a greater time interval and also by more than 5% of the mean. Thus, PT and APTT testing on automated instrumentation is very precise, but occasional inaccurate single measurements could lead to errors in diagnosis or therapy.