Background and objective Perioperative hypothermia has been found to impair the coagulation cascade and to increase blood loss and transfusion requirements. The effect of concomitant in vitro heparinization on coagulation during hypo- and hyperthermic conditions has not been well defined.

Methods In the present study, activated partial thromboplastin time was examined in vitro at 33°C, 35°C, 37°C, 39°C and 41°C in normal human plasma in response to unfractionated heparin.

Results Hypothermia ≤ 35°C prolonged activated partial thromboplastin time by 10% compared with test temperatures at 37°C (P < 0.05). Hyperthermia alone had no effect. Unfractionated heparin (0.1–0.4 IU mL−1) increased activated partial thromboplastin time in a dose-dependent manner (by 189% at 0.4 IU mL−1, P < 0.05). Test temperatures of 33°C and 41°C increased heparin-induced prolongation of activated partial thromboplastin time. At a heparin concentration of 0.4 IU mL−1, hypothermia (33°C) and hyperthermia (41°C) prolonged partial thromboplastin time by 12% and 22%, respectively, compared with normothermic test temperature of 37°C within the heparin group (P < 0.05).

Conclusions These observations suggest that both hypo- and hyperthermia increase the response to heparin in vitro. Further studies are needed to identify the effect of patient’s body temperature on heparin activity and bleeding tendency in vivo.