Activated protein C (APC) normally has an anticoagulant effect. Some patients with recurrent venous thrombosis do not show the expected anticoagulant effect when activated protein C (APC) is added to clotting tests, and these persons were termed  "resistant" to the anticoagulant effect of APC. Many of these patients have an hereditary disorder (autosomal dominant inheritance) related to a mutation in factor V (Factor V Leiden) which resists proteolysis by APC when activated to factor Va.

APC resistance ratio =

= (modified aPTT with APC added) / (modified aPTT without APC added)


normalized APC resistance ratio =

= (patient's APC resistance ratio) / (APC resistance ratio for pooled normal plasma)



• The lower limit of the reference range for the APC resistance ratio varies with each laboratory, but is typically 2.0-2.2

• An APC resistance ratio less than the lower limit for the reference range is seen in patients with APC resistance.

• A normalized APC resistance ratio < 0.84 is seen with APC resistance.

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