ACTIVATED PROTEIN C (APC) RESISTANCE TEST - plasma

---

Specimen: 4.5 mL blood in 0.5 mL citrate.

Method: Coagulation tests in the presence and absence of activated protein C; the clotting times are recorded. The ratio of clotting times with and without activated protein C is determined.

Reference Interval: APTT-based method.

Normal:	> 2.2
Equivocal:	2.0-2.2
Abnormal:	< 2.0

Application: Investigation of tendency to venous thromboembolism: unexplained, recurrent, or with a positive family history. The test has high sensitivity and specificity and is an adequate initial test, except for patients receiving heparin or warfarin and those with other abnormalities of coagulation. In these circumstances, DNA testing for detection of the abnormal factor V gene is also available; see MOLECULAR GENETICS- INDIVIDUAL GENETIC DISORDERS – FACTOR V LEIDEN MUTATION.

Interpretation: Activated protein C resistance is due to an inherited disorder of the (coagulation) factor V molecule, and is associated with venous thromboembolism. The disorder is characterised by a low anticoagulant response of plasma on addition of activated protein C. The abnormality has been reported in 20-60% of patients with venous thromboembolism, although thrombosis usually only occurs when other risk factors are present. A single gene mutation (factor V Leiden) has been described in >90% of APC resistant patients and can be detected by DNA testing: see MOLECULAR GENETICS - INDIVIDUAL GENETIC DISORDERS – FACTOR V LEIDEN MUTATION. Aquired APC resistance may occur in pregnancy and in the presence of inflammation.

Reference: Svesson PJ and Dahlbäck B. N Engl J Med 1994; 330: 517-522. Dahlbäck B. Thromb Haemostas 1995; 73: 739-742.

---