Measurement of prothrombin time (PT) and activated partial thromboplastin time (APTT) on canine citrated plasma samples following different storage conditions

Samples from 75 clinically ill dogs were utilised in the study. APTT and PT tests were performed immediately on fresh citrated plasma samples (Fresh). The remaining plasma was stored at -20 degrees C for less than 4 months (n=36 samples) or between 4 and 7 months (n=39 samples). In batches of five, frozen samples were thawed rapidly and APTT and PT tests were performed on the thawed samples immediately (0RT) and after storage at room temperature (23 degrees C, range: 22-25 degrees C) for 24h (24RT) and 48h (48RT). The median APTT value from the (0RT) samples was significantly longer than that obtained from fresh samples (15s vs. 13.2s) but the PT value was not statistically different (7.8s vs. 7.6s). The median APTT (15s) and PT (7.5s) results from the (24RT) samples were not statistically different to those from the (0RT) samples (APTT: 15s, PT: 7.6s) but both tests were significantly longer (APTT: 16.5s, PT: 9.2s) from the (48RT) samples. We concluded that long term batching and freezing of clinical samples at -20 degrees C is acceptable for measurement of PT but not of APTT. We demonstrated that APTT and PT results do not change following storage of samples at room temperature for 24h but storage for 48h may lead to statistically and clinically significant changes (values at least 25% higher than the high value of the laboratory's reference interval) in both clotting times.