Hypothermia prolongs clotting times when the tests are performed at hypothermic temperatures, in contrast to standard clinical tests performed at 37 degrees C. The relative impact of hypothermia on plasma clotting factor activity was investigated by determining the specific clotting factor deficiencies required to produce an equivalent effect. Clotting factor concentration curves were constructed for clotting factors II, V, and VII through XII using assayed reference plasma (ARP) diluted with specific factor-deficient plasmas (FDP). Prothrombin times and partial thromboplastin times were measured as appropriate for each factor at test temperatures ranging from 37 degrees to 25 degrees C using a modified fibrometer. The clotting times for each temperature with undiluted ARP were compared with the clotting times at 37 degrees C obtained with FDP dilution. Hypothermia at temperatures below 33 degrees C produces a coagulopathy that is functionally equivalent to significant (< 50% of normal activity) factor-deficiency states under normothermic conditions, despite the presence of normal clotting factor levels.