Activated protein C (APC) resistance test - plasma

Last updated: Wednesday, 31, March, 2004

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Item Process
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. Guide only, check with laboratory.
Normal: > 2.2
Equivocal: 2.0-2.2
Abnormal: < 2.0

RWT-based method. Guide only; check with laboratory.
Normal: > 1.8
Equivocal: 1.6-1.7
Abnormal: < 1.6.

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.

Interpretation

Activated protein C resistance is associated with an increased risk of venous thromboembolism.

However, as the condition is detectable in ~5% of all individuals, an abnormal APCR result does not necessarily indicate a future thrombosis. 

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 additional 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.

Acquired APC resistance may occur in pregnancy and in the presence of inflammation.

See also Factor V Leiden.