Prothrombin time and activated partial thromboplastin time can be performed on the first tube.

The current practice standard for coagulation studies on venous blood specifies the use of the second (or subsequent) tube for testing. Unless other tests are ordered, the first tube is discarded. This practice, derived from early experience using nondisposable materials, gained support from a report based on modern phlebotomy methods but inexperienced phlebotomists. For our study, paired blood specimens were collected by experienced phlebotomists from 175 outpatients whose physicians had ordered coagulation tests. Prothrombin time (PT) and activated partial thromboplastin time (APTT) were performed on the first and second tubes. Similar values were obtained for PT and APTT. (PT: n = 174; mean = 15.36 seconds; mean difference = 0.10 seconds; y = 1.02x - 0.28; t = 0.24; P>.81; r = 0.995; APTT: n = 160; mean = 38.25 seconds; mean difference = 0.48 seconds; y = 1.03x - 0.71; t = 0.25; P=.80; r = 0.993.) For coagulation tests on specimens collected by experienced phlebotomists, the first tube yields accurate results and need not be discarded.