Activated Protein C Resistance (APCR)
Screening test for the factor V Leiden gene mutation and other causes of activated protein C resistance.
Additional Teat Information:
Activated protein C (aPC), in a complex with protein S, inactivates procoagulant factors Va and VIIIa by proteolytic cleavage at specific arginine residues.7,10,11 This serves to control coagulation and limit the extent of thrombus formation. The functionality of the aPC inhibitory system in a given individual can be assessed through an in vitro clotting assay. Addition of aPC to a patient’s plasma serves to extend the activated partial thromboplastin time (aPTT) for individuals who are sensitive to aPC. Individuals are considered to be aPC resistant when addition of aPC fails to extend the time to clot formation in this assay. More than 95% of cases of aPC resistance are caused by a specific polymorphism in the factor V gene that is referred to as factor V Leiden.8 This single point mutation results in a substitution of glutamine for arginine at amino acid number 506 of factor V. Arginine number 506 is an aPC cleavage site of normal factor V, making factor V Leiden resistant to inactivation by aPC. Heterozygous factor V Leiden mutation occurs in 3% to 7% of persons of Northern European descent. However, this mutation is essentially absent in Asian, African, and Mediterranean populations.8 Individuals with heterozygous factor V Leiden have a three- to tenfold increased risk of thrombosis.8 The risk of thrombosis is approximately 80-fold increased in individuals who are homozygous for factor VLeiden.8 The incidence of factor V Leiden in unselected individuals experiencing their first venous thrombosis or pulmonary embolism is approximately 20%. In patients with a family history of thrombophilia, the likelihood of factor VLeiden being the cause reaches as high as 50%.8 Risk of thrombosis is further increased if the factor V Leiden mutation is accompanied by other prothrombotic conditions (ie, antithrombin, protein C or protein S deficiency, prothrombin G20210, pregnancy, oral contraceptive usage, prolonged immobilization, or surgery). While factor V Leiden is, by far, the most common genetic cause of aPC resistance, other polymorphisms can produce this condition.9 aPC resistance can occur due to an elevated factor VIII level, in itself a risk factor for thrombosis.9 Factor V Cambridge, a very rare mutation at another arginine residue of factor V, can produce aPC resistance.7,8 The HR2 haplotype, also referred to as A4070G mutation, is another rarely observed genetic cause of aPC resistance.7
Blue-top (sodium citrate) tube
Do not draw from an arm with a heparin lock or heparinized catheter. Ideally, the patient should not be on anticoagulant therapy. Avoid warfarin (Coumadin®) therapy for two weeks prior to the test and heparin (direct Xa and thrombin inhibitor therapies) for about three days prior to testing.
Citrated plasma samples should be collected by double centrifugation. Blood should be collected in a blue-top tube containing 3.2% buffered sodium citrate.1 Evacuated collection tubes must be filled to completion to ensure a proper blood to anticoagulant ratio.2,3 The sample should be mixed immediately by gentle inversion at least six times to ensure adequate mixing of the anticoagulant with the blood. A discard tube is not required prior to collection of coagulation samples, except when using a winged blood collection device (ie, “butterfly”), in which case a discard tube should be used.4,5 When noncitrate tubes are collected for other tests, collect sterile and nonadditive (red-top) tubes prior to citrate (blue-top) tubes. Any tube containing an alternate anticoagulant should be collected after the blue-top tube. Gel-barrier tubes and serum tubes with clot initiators should also be collected after the citrate tubes. Centrifuge for 10 minutes and carefully remove 2/3 of the plasma using a plastic transfer pipette, being careful not to disturb the cells. Deliver to a plastic transport tube, cap, and recentrifuge for 10 minutes. Use a second plastic pipette to remove the plasma, staying clear of the platelets at the bottom of the tube. Transfer the plasma into a transpak frozen purple tube with screw cap. Freeze immediately and maintain frozen until tested.
Gross hemolysis; clotted specimen; frozen specimen thawed in transit; tubes <90% full; improper labeling; specimen collected in tube other than 3.2% citrate