Protein S and Protein C are vitamin K dependent and are decreased in patients receiving coumadin. In a patient receiving coumadin, comparison of protein S and protein C levels to other vitamin K dependent coagulation factors can indicate if a deficiency in protein S or C is likely.


ratio of protein C to Factor 7 =

= (per cent normal activity protein C) / (per cent normal activity Factor 7)


ratio of protein S to Factor 10 =

= (per cent normal activity protein S) / (per cent normal activity Factor 10)



• a ratio of protein C-to-Factor 7 < 0.7 is consistent with protein C deficiency

• a ratio of protein S-to-Factor 10 < 0.7 is consistent with protein S deficiency



This method can be misleading, especially in borderline deficiencies. Patients should be on stable dosing of anticoagulant. It is better to stop coumadin therapy and to perform testing after sufficient time (at least 1 week) for reversal of coumadin effect.


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