STABILITY OF PLASMA FOR ADD-ON PT AND APTT TESTS

We conducted studies to determine at what time point an add-on prothro mbin time (PT) or activated partial thromboplastin time (APTT) test ca n be honored on specimens that have been received in the laboratory ho urs earlier without yielding results with clinically significant diffe rences from those if the test had been performed on the original unsto red plasma. PT and APTT tests were performed on blood samples from 20 healthy subjects, 30 patients receiving warfarin, and 30 patients rece iving heparin anticoagulation therapy The tests were performed on plas ma 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 ma jority of plasma samples from healthy subjects and from patients recei ving oral anticoagulant therapy tended to become shorter on storage. H owever, the difference in PT values was small and had no clinical sign ificance. In most cases, the APTT values for the stored plasma from he althy 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 res ult for the original sample, all others had APTT results less than 1.2 seconds longer than the original values. In patients receiving hepari n, 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 f or heparinized samples with high APTT values. Unlike samples from heal thy subjects, there was no obvious trend of time-related prolongation of the APTT in heparinized plasma. These results led us to believe tha t within an 8-hour period and with plasma on spun-down cells at room t emperature, add-on tests for PT and APTT could be performed with resul ts similar to what would be obtained from testing unstored samples.