Activated Protein C Resistance (APCR)

CPT: 85307
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Test Details

Synonyms

  • APCR

Use

Screening test for the factor V Leiden gene mutation and other causes of activated protein C resistance

Limitations

This assay detects thrombotic risk factors other than factor V Leiden. This test should not be performed on individuals with extended baseline aPTT values due to the presence of lupus anticoagulants.6 This test should not be used for patients receiving thrombin inhibitors such as hirudin and argatroban.6 In some cases, this test will fail to distinguish individuals who are heterozygous for the factor V Leiden mutation from normals.7 Genetic testing may be required to distinguish between heterozygous and homozygous factor V Leiden mutation.7 Elevated factor VIII levels, as can be seen in acute phase reaction, can normalize the aPTT and effectively reduce the anticoagulant effect of aPC.7 Abnormal results can also be seen in pregnancy, especially during the third trimester, due to decreased levels of protein S and increased levels of factors V and VIII.7

Methodology

This assay measures the effect of adding aPC on the activated partial thromboplastin time (aPTT) of the patient sample.8,9 An aPTT is performed twice, once with and again without added aPC. The ratio of the two clotting times is reported. The patient sample is diluted with factor V-deficient plasma. This serves to correct for any factor deficiencies in the patient sample and allows the test to be used for patients on oral anticoagulants. The reaction mixture also contains a heparin neutralizer to allow this test to be performed on patients receiving heparin therapy.

Additional 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

Specimen Requirements

Specimen

Plasma, frozen

Volume

2 mL

Minimum Volume

1 mL

Container

Blue-top (sodium citrate) tube

Patient Preparation

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.

Collection

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 LabCorp PP transpak frozen purple tube with screw cap (LabCorp N° 49482). Freeze immediately and maintain frozen until tested. To avoid delays in turnaround time when requesting multiple tests on frozen samples, please submit separate frozen specimens for each test requested.

Please print and use the Volume Guide for Coagulation Testing to ensure proper draw volume.

Storage Instructions

Freeze

Causes for Rejection

Gross hemolysis; clotted specimen; frozen specimen thawed in transit; tubes <90% full; improper labeling; specimen collected in tube other than 3.2% citrate

Clinical Information

Special Instructions

If the patient's hematocrit exceeds 55%, the volume of citrate in the collection tube must be adjusted. Refer to Coagulation Collection Procedures for directions.

Footnotes

1. Adcock DM, Kressin DC, Marlar RA. Effect of 3.2% vs 3.8% sodium citrate concentration on routine coagulation testing. Am J Clin Pathol. 1997 Jan; 107(1):105-110. 8980376
2. Reneke J, Etzell J, Leslie S, Ng VL, Gottfried EL. Prolonged prothrombin time and activated partial thromboplastin time due to underfilled specimen tubes with 109 mmol/L (3.2%) citrate anticoagulant. Am J Clin Pathol. 1998 Jun; 109(6):754-757. 9620035
3. National Committee for Clinical Laboratory Standardization. Collection, Transport, and Processing of Blood Specimens for Coagulation Testing and General Performance of Coagulation Assays; Approved Guideline. 5th ed. Villanova, Pa: NCCLS; 2008. Document H21-A5:28(5).
4. Gottfried EL, Adachi MM. Prothrombin time and activated partial thromboplastin time can be performed on the first tube. Am J Clin Pathol. 1997 Jun; 107(6):681-683. 9169665
5. McGlasson DL, More L, Best HA, Norris WL, Doe RH, Ray H. Drawing specimens for coagulation testing: Is a second tube necessary? Clin Lab Sci. 1999 May-Jun; 12(3):137-139. 10539100
6. Van Cott EM, Laposata M. Coagulation. In: Jacobs DS, DeMott WR, Oxley DK, eds. Laboratory Test Handbook With Key Word Index. 5th ed, Hudson, Ohio: Lexi-Comp; 2001: 327-358.
7. Adcock DM, Bethel MA, Macy PA. Coagulation Handbook. Aurora, Colo: Esoterix−Colorado Coagulation; 2006.
8. Van Cott EM, Soderberg BL, Laposata M. Activated protein C resistance, the factor V Leiden mutation, and a laboratory testing algorithm. Arch Pathol Lab Med. 2002 May; 126(5):577-582. 11958664
9. Graf LL, Welsh CH, Qamar Z, Marlar RA. Activated protein C resistance assay detects thrombotic risk factors other than factor V Leiden. Am J Clin Pathol. 2003 Jan; 119(1):52-60. 12520697
10. Zwicker J, Bauer KA. Thrombophilia. In: Kitchens CS, Alving BM, Kessler CM, eds. Consultative Hemostasis and Thrombosis. Philadelphia, Pa: WB Saunders Co; 2002:181-196.
11. Triplett DA. Thrombophilia. In: McClatchey KD, ed. Clinical Laboratory Medicine. 2nd ed. Philadelphia, Pa: Lippincott Williams and Wilkins; 2002:1050-1056.

LOINC® Map

Order Code Order Code Name Order Loinc Result Code Result Code Name UofM Result LOINC
117762 Activated Protein C Resistance 13590-5 117763 Act.Prt.C Resist. ratio 13590-5
Reflex Table for Act.Prt.C Resist.
Order Code Order Name Result Code Result Name UofM Result LOINC
Reflex 1 117764 Comment 117764 Comment N/A

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