Activated Protein C Resistance (APC-R), Plasma

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1) Why are normally no clots formed in the human body?

Humans have both pro-coagulant (clot formation) and anti-coagulant (clot resolution or breakdown) systems in the body. Whenever there is any form of bleeding, immediately pro-coagulant mechanisms (clotting factors) come into play and they stop the bleeding by clot formation. On other hand, excess activation of clot formation is kept in check by anti-coagulant factors. Thus a delicate balance is always maintained between these two opposite systems in the body depending upon the body’s requirement. Excess of pro-coagulant or deficiency of anti-coagulant factors can lead to excessive clot formation and thrombo-emolic phenomenon in the body. Another spectrum is a deficiency of clotting factors, which leads to bleeding episodes.

2) What is Protein C?

Protein C is a part of anti-coagulant system. It is a vitamin-K dependant, naturally occurring anticoagulant in the human body which is synthesized in the liver and circulating in the blood at a concentration of approx 5µg/mL. Whenever clots are formed, Protein C undergoes proteolytic cleavage to form APC which down-regulates pro-coagulant systems by proteolytic inactivation of activated clotting factors V and VIII. Another protein called Protein S helps Protein C in this process.

3) What is the basic abnormality in patients having APC resistance?

The majority of patients have a specific point mutation in the procoagulant factor V gene. This mutation alters the process of proteolytic cleavage and the formation of APC. As a result, factor V becomes resistant to APC mediated inactivation. The most common mutation is Factor V Leiden mutation, which is also a common cause of APC resistance.

 4) What are the special instructions for this sample collection?

Preanalytical conditions play a very important role in this testing for getting reliable and accurate results. This is a labile analyte. Plasma needs to be separated by centrifugation at the earliest possible to avoid errors in reporting.

 5) What are the advantages of this testing?

This is a highly sensitive and specific test for the diagnosis of APC resistance, which is most commonly due to the factor V Leiden mutation.

 6) Does this testing have any limitations?

Certain interference factors may affect ratio values of APC which cannot be clearly attributed to a particular genotype or may lead to clotting times that exceed the maximum admitted detection time of that particular instrument. In such cases, when clinically indicated, further investigation should be done with the help of DNA-based testing - PCR for the factor V Leiden mutation and determination of individual factors. Also, this test will not detect patients with acquired APC resistance.

 7) Is it possible that the results are positive but the patient does not have APC resistance?

In medical language, we label such cases as false positives. This may be due to Aprotinin or Protamine. Also for accurate measurement of functional assay levels of APC, the patient should discontinue any form of oral anticoagulants for at least 2 weeks prior to testing.

8) Is it possible that the results are negative but the patient is having APC resistance?

In medical language, we label such cases as a false negative. This may be due to low levels of anticoagulant Factor V (<50%), excess heparin, direct thrombin inhibitors like hirudin, argatroban, bivalirudin, and dabigatran, or direct factor Xa inhibitors like rivaroxaban, apixaban, and edoxaban.

 9) Do I need to add any other test?

This test is best done as a part of thrombophilia profile because it allows a better interpretation of the assay results and gives an overall idea of the body’s coagulation system. This profile includes APC resistance, antigen, and activity of Protein C and Protein S, Antithrombin III and Lupus anticoagulant, homocysteine, anti-cardiolipin antibody IgG & IgM, phospholipids antibody IgG & IgM, and Factor VIII activity.

10) What are the complications in patients having APC resistance?

Patients may complain of blood clots formation in veins, deep vein thrombosis in legs, stroke, myocardial infarction, pulmonary embolism, and death.

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