Stability of plasma for add-on PT and APTT tests

We conducted studies to determine at what time point an add-on prothrombin time (PT) or activated partial thromboplastin time (APTT) test can be honored on specimens that have been received in the laboratory hours earlier without yielding results with clinically significant differences from those if the test had been performed on the original unstored plasma. PT and APTT tests were performed on blood samples from 20 healthy subjects, 30 patients receiving warfarin, and 30 patients receiving heparin anticoagulation therapy. The tests were performed on plasma prepared initially after the samples were obtained. The same tests were assayed on plasma that had been left on spun-down blood cells at room temperature for 2, 4, and 8 hours. We found that the PT of the majority of plasma samples from healthy subjects and from patients receiving oral anticoagulant therapy tended to become shorter on storage. However, the difference in PT values was small and had no clinical significance. In most cases, the APTT values for the stored plasma from healthy subjects tended to increase with time. Except in one specimen in which the 8-hour add-on APTT was 1.2 seconds longer than the APTT result for the original sample, all others had APTT results less than 1.2 seconds longer than the original values. In patients receiving heparin, the differences in APTT values between the initial and add-on tests were larger than those observed for healthy subjects. However, those differences are not beyond what we would accept for duplicate checks for heparinized samples with high APTT values. Unlike samples from healthy subjects, there was no obvious trend of time-related prolongation of the APTT in heparinized plasma. These results led us to believe that within an 8-hour period and with plasma on spun-down cells at room temperature, add-on tests for PT and APTT could be performed with results similar to what would be obtained from testing unstored samples.