APC resistance (Factor V Leiden)

Physiology / pathophysiology
In the coagulation system, there is a balance between coagulation and anticoagulation (haemostatic balance) under physiological conditions that is maintained by the finely regulated interplay of activators and inhibitors. Together with its cofactor protein S, protein C – which is activated by thrombin – plays a special role in this.

Activated protein C (APC) acts as a central inhibitor by limiting further thrombin formation through the proteolytic cleavage of coagulation accelerator factors Va and VIIIa, thereby preventing excessive coagulation.

According to the present state of knowledge, resistance to APC is the most common hereditary risk factor for thrombophilia.

In > 95 % of all cases, the cause of APC resistance is a point mutation in the gene for coagulation factor V, called the Factor V Leiden mutation. Other, much rarer sequence variations in the factor V gene, including Factor V Cambridge, have been described, and they also lead to APC resistance.

A consequence of these mutations is a change in the amino acid sequence at the APC binding site for factor V. APC can no longer „dock” with it and, as a result, the inactivation of factor Va is significantly delayed. This results in hypercoagulability.

APC resistance is often associated with other hereditary thrombophilia risk factors, such as prothrombin gene mutation, antithrombin deficiency, protein C and protein S deficiency and hyperhomocysteinemia. Especially in younger patients, therefore, extended thrombophilia diagnostics should always be carried out to asses the risks to the patient as fully as possible.

Acquired APC resistance can be caused by changes in the factors VIII, protein C and protein S — such as during pregnancy, from treatment with oral contraceptives, during and following operations, or due to immunological disorders. This must be taken into account when interpreting the APC resistance results.

Functional detection of APC resistance as a screening test is conducted by determining aPTT, with and without the addition of activated protein C, and assessing the ratio of the two measurements. At a ratio of > 2.0, APC resistance can largely be ruled out.

A ratio of ≤ 1.2 indicates that the presence of a homozygous defect is highly probable.

In borderline results (ratios between 1.4 and 1.9), molecular analysis of Factor V Leiden should be performed to confirm the diagnosis, since it is only possible to distinguish between heterozygous and homozygous forms with certainty by means of a mutation analysis. This differentiation is absolutely necessary for assessing the risk of thrombosis and a possible need for therapy.

A further advantage is that, in contrast to the functional APC resistance test, the molecular biological test can also be performed without falsifying the results when oral anticoagulation is being taken, when there is a high-titre lupus anticoagulant and other antiphospholipid antibodies and when there is acute thrombosis.

Thromboembolisms of uncertain origin, especially in patients under the age of 45, recurrent thromboembolisms, thrombosis in unusual locations, a tendency towards spontaneous abortions and intrauterine foetal death

Disturbing and influencing variables
Heparin in high concentrations, therapy with hirudin, argatroban and other direct thrombin inhibitors (false normal ratio possible)

Test material
APC resistance test: 3 ml of citrate blood
Factor V Leiden mutation analysis: 5 ml of EDTA blood

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