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BACKGROUND: Hypothermia may be accidental or therapeutic. Therapeutic hypothermia is increasingly used as treatment for various conditions, e.g., neuroprotection after cardiac arrest. Hypothermia leads to an impairment of the coagulation system, but the degree of impairment has been difficult to determine. Most studies have been performed on plasma instead of whole blood. We therefore evaluated whole blood investigating the effects of hypothermia on the coagulation system over a wide range of temperatures (25-40 degrees C). METHODS: Blood was drawn from six healthy volunteers into citrated test tubes. Samples were then placed in water baths with temperatures ranging from 25 to 40 degrees C for 30 min before the coagulation system was studied using rotational thromboelastometry. A contact activator (Ellagic acid) was used for initiation of coagulation. Clotting time, clot formation time, a angle, and maximum clot strength were measured. All tests were run for 60 min and they were performed at the same temperature as the temperature in the water bath. RESULTS: Coagulation was increasingly impaired with decreasing temperatures in the temperature range studied. All variables measured were significantly impaired in a stepwise pattern (P < 0.0001). CONCLUSIONS: Evaluation using a whole blood analysis shows that hypothermia progressively impairs the coagulation system. (Less)