Some species differences in the false prolongation of prothrombin times and activated partial thromboplastin times in toxicology

Changes in plasma activated partial thromboplastin times (APTT) and prothrombin times (PT) in mice, rats, rabbits, dogs, monkeys and human were examined for up to 96 h at storage temperatures of 4 and 25°C. Prolongation of APTT in rats was rapid and marked, with times doubling within 24 h post-sampling. Plasma APTT of human and monkey were also affected, but to a lesser extent. No effect was observed in mice, rabbits and dogs. On the other hand, the magnitude of PT changes was much smaller than that observed with APTT in all species. No significant differences were noted between the results from samples stored at 4°C or 25°C for either test.The false prolongation of APTT is clearly undesirable in a toxicity study, especially in rats. It is important therefore to minimise these changes by performing this test under strict time-controlled conditions.