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ACTIVATED PROTEIN
C RESISTANCE
(APC RESISTANCE)
AND MUTATIONS IN FACTOR V
MOLECULAR BASIS:
When thrombin (IIa) is generated, it has
both procoagulant activities and anticoagulant activities. Excess thrombin
is washed downstream where it binds to thrombomodulin (TM) on endothelial
cells of the vessel wall. Protein C from plasma binds to the IIa/TM
complex and is cleaved to its active form, termed "activated Protein C"
(APC). APC is one of the most physiologically important anticoagulants
as it selectively degrades the coagulation cofactors, Va and VIIIa to limit
thrombin generation, fibrin formation and blood clotting in vivo.
It has been shown that APC resistance results from a mutant Factor Va molecule, termed Factor V Leiden. This molecule has a specific point mutation (Arg506 -> Gln506) which cannot be degraded by APC in more than 90% of all APC resistant patients2. The defective Va is able to clot as normal and clotting continues because of this resistance to inactivation by APC. Other APC cleavage sites in the Factor Va molecule are potential mutation sites. These sites include: Arg306 -> Gln306 and Arg679 -> Gln679. The other 10% of APC resistance includes Acquired APC Resistance and these secondary sites.
Recent studies have shown the occurrence of APC resistance to vary from 2 - 16% depending on the population studied. This data suggests that screening protocols for hereditary thrombotic disorders should include testing for APC resistance as an important genetic risk factor. Hypercoagulability has been explained by hereditary deficiencies of Protein C, Protein S and Antithrombin in only 9 to 21% of thrombotic cases in patients without the usual risk factors for thrombosis (i.e., cancer, recent surgery, lupus anticoagulant)3. Several investigators have reported APC resistance abnormalities in 20-50% of all cases of venous thromboses2. The Factor V Leiden mutation is most common in Caucasian populations. There are also strong interactions with Protein C deficiencies, Protein S deficiencies, and the prothrombin 20210A gene mutation (which allows excess prothrombin to be manufactured). See our Coag Capsule on the Prothrombin Gene Mutation for more information.
CLINICAL APPLICATIONS:
An abnormal result identifies a major
risk factor and provides an explanation for thrombotic episodes.
Furthermore, relatives of affected patients should be tested as they may
be at high risk for developing thrombosis. Testing for APC resistance is
recommended in the following situations:
- investigation of familial thrombosis, i.e, thrombophilia
- venous thrombotic events
- recurrent TIAs or strokes, particularly in those patients under 50
- patients being considered for anticoagulant therapy
- women considering using oral contraceptives
The APC Resistance assay measures the anticoagulant response to APC in a modified APTT test system containing Factor V deficient plasma. Modifications from the original methodology allow for testing of patients on coumadin and/or heparin. APC resistance is reported as "negative" or "positive". A negative result means the Factor V Leiden mutation is NOT present. A positive result may indicate: (a) a heterozygous or homozygous Factor V Leiden mutation; (b) another mutation site (explained above); or (c) an acquired APC Resistance due to other causes.
Factor V Leiden is measured using Fluorescence Resonance Energy Transfer (FRET) technology to specifically identify the Arg506 -> Gln506 mutation. The test does not identify all APC resistant patients; only those with the Arg506 -> Gln506 mutation. The other two other mutations of the Factor V molecule are identified by using the APC Resistance assay.
ORDERING INFORMATION:
A cost-effective approach to screening
for this defect is to first perform the APTT based assay which will detect
virtually all cases of APC resistance. Using FRET technology, the
presence of the Factor V Leiden mutation can be confirmed as heterozygous
or homozygous in approximately 90% of patients with the APC resistant phenotype.
APC RESISTANCE: To assure accuracy of test results, the APC Resistance test must be performed on fresh frozen platelet poor plasma.
Requirement: Two 1.0 ml aliquots of citrated plasma (blue top tube).
Cost: $75.00
FACTOR V LEIDEN MUTATION:PLEASE NOTE:
Requirement: 5 ml of EDTA whole blood (purple top vacutainer tube), collected and shipped at room temperature. Sample must be received within 24 hours of draw. Notification of the laboratory on the day the sample is shipped is required. Do not ship on Friday, Saturday, or the day before a holiday.
Cost: $100.00
For your patient's convenience, you can draw samples for both assays and send them to us. We will perform the APC Resistance initially and reflex to the Factor V Leiden only if you provide written instructions on the test requisition to confirm that you want this additional testing.
If you order a Factor V without specifying Leiden, the lab will order a Factor V Activity. This test is primarily used for bleeding disorders, not clotting disorders.
Please Call HEMEX Client Services
for sample pick up at (800) 999-2568
or in the greater Phoenix area at
(602) 997-9161
References:
1. Bertina R., Reitsma PH, Rosendaal FR, and Vandenbroucke J. Resistance to Activated Protein C and Factor V Leiden as Risk Factors for Venous Thrombosis. Thrombosis and Haemostasis 1995;74(1):449-453.2. Dahlbäck B. Resistance to Activated Protein C, the Arg506 to Gln Mutation in the Factor V Gene, and Venous Thrombosis. Thrombosis and Haemostasis 1995;73(5):739-742.
3. Griffin JH., Evatt B, Wideman C, and Fernández JA. Anticoagulant Protein C Pathway Defective in Majority of Thrombophilic Patients. Blood 1993;82(7):1989-1993.
4. Ridker PM., Miletich JP, Stampfer, M J, Goldhaber SZ, Lindpainter K, Hennekens CH. Factor V Leiden and Risks of Recurrent Idiopathic Venous Thromboembolism. Circulation 1995;92(10):2800-2802.