Detection of protein C deficiency during oral anticoagulant therapy--use of the protein C:factor VII ratio.

Laboratory diagnosis of protein C deficiency is complicated by the fact that many patients referred for investigation are already being treated with oral anticoagulants. Protein C and factor VII have similar half lives and by using the protein C:factor VII ratio we hoped to be able to detect protein C deficiency in patients receiving oral anticoagulants. We have studied activity levels of protein C and factor VII to produce protein C:factor VII activity ratios in 105 patients receiving oral anticoagulants, 42 normal subjects, and nine patients with known inherited protein C deficiency. The mean ratios for these groups were 1.38, 1.12 and 0.63 respectively. In patients receiving oral anticoagulants, the protein C level showed a poor correlation with the international normalized ratio (INR) value. Reference ranges for protein C at different levels of anticoagulation were therefore considered unsuitable for the identification of inherited protein C deficiency. Five patients with inherited protein C deficiency were studied with and without oral anticoagulation; their protein C:factor VII ratios remained relatively unchanged, despite alterations in the level of the individual proteins. These results suggest that measurement of the protein C:factor VII ratio may help to identify patients with inherited protein C deficiency whilst on oral anticoagulants.