The plasma levels of protein C were investigated in 54 type I diabetic patients without retinopathy and in 14 diabetic patients with diabetic retinopathy and compared with the findings in 35 sex- and age-matched healthy control subjects. In the total group of type I diabetic patients, protein C was significantly less than in the controls. The lowest levels of protein C were found in diabetic patients with the poorest metabolic control. Protein C levels showed a significant negative correlation with the blood glucose levels, but they were not correlated with hemoglobin A1c (HbA1c). Although patients with retinopathy showed the least decrease of the plasma level of protein C among the diabetic subjects, the ratio of protein C to factor II was significantly decreased compared with the control subjects. Because the levels of coagulation factor II were not reduced in diabetic patients, the reduction of protein C seems to be caused, not by reduced synthesis in the liver, but more likely by an increased clearance from the blood plasma. The decrease of protein C in the plasma of type I diabetic patients indicates an abnormal, probably hypercoagulable, hemostatic situation in this disorder.

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