Assay of protein C in human plasma: comparison of amidolytic, coagulation, and immunochemical assays.

We studied functional protein C activity, both anticoagulant and amidolytic, as well as protein C antigen in 30 normal subjects, several members of a family with congenital protein C deficiency, 18 patients with severe preeclampsia, 27 patients with coronary heart disease, including 15 patients with myocardial infarction and 12 with angina pectoris, 20 patients on stable oral anticoagulant therapy (thrombotest values: 3-12%) and three patients with disseminated intravascular coagulation. Protein C values measured by the coagulant assay were compared to those obtained with amidolytic and immunochemical assays. In all the groups studied, the activity assays (amidolytic and coagulant) correlated significantly with each other as well as with the immunochemical assay. In patients on oral anticoagulant therapy the coagulant assay gave lower protein C values than amidolytic and immunochemical assays. A good correlation was found between immunological and amidolytic protein C assays (r=0.90, p less than 0.001), immunological and coagulant protein C assays (r=0.93, p less than 0.001), and amidolytic and coagulant protein C assays (r=0.95, p less than 0.001) in all the samples studied without including the protein C values of patients on oral anticoagulant therapy. These results allow us to recommend the functional protein C coagulant assay in patients on stable oral anticoagulant therapy because only this assay evaluates the "in vivo" protein C function in these patients.