Thrombophilia

Who should have thrombophilia screening? : When should thrombophilia screening be done? : What tests should be done?: : : : : :

It has been recognised for some time that certain individuals appear to have an increased tendency to thrombosis, venous thrombosis such as deep venous thrombosis or pulmonary embolism or arterial thrombosis. Inherited abnormalities which predispose to thrombosis have been known about for some time. These can be due to inherited deficiencies or abnormalities of natural inhibitor proteins of the coagulation system. These inhibitors exist to control the rate of formation of a blood clot. A second cause of inherited thrombosis is deficiency of proteins in the fibrinolytic system, these proteins break down blood clots once they are formed. Deficiencies of the fibrinolytic system are very rare and are not usually tested for.

Protein C.

Protein C is part of the anticoagulant regulatory mechanism. It is converted to activated protein C (APC) by thrombin in the presence of thrombomodulin. APC inactivates activated factors V and VIII. Protein C deficiency has been shown to be a risk factor for thrombosis.

Antithrombin

Antithrombin (AT) is a major inhibitor of blood coagulation and is essential for effective heparin therapy. AT inhibits the coagulation proteases including II a, X a , IX a and XI a. AT deficiency is associated with a high risk of thrombotic disorders.

Free Protein S

Protein S is a vitamin K dependent cofactor for the anticoagulant activity of activated protein C (APC). Two forms of protein S are present in plasma : free protein S (40%) and protein S linked to the C4b-binding protein (60%). Only the free form has functional cofactor activity. Protein S deficiency may be hereditary or acquired. It has been associated with a high risk of developing venous thromboembolism especially in young people. As only the free form of Protein S has the cofactor activity it is only this form that is measured.

APC Resistance Assay

Protein C is a naturally occurring inhibitor of blood coagulation, acting on activated factor V and VIII. The condition whereby a patient’s plasma does not produce the appropriate anticoagulant response to activated protein C (APC) is termed APC resistance. This is caused by the VQ506 gene mutation which produces factor V Leiden, a factor V molecule which is resistant to cleavage by activated protein C.

Factor V Leiden mutation

The identification of Factor V Leiden mutation is carried out using PCR technology. This method is used to identify the genotype of the abnormality. PCR testing is carried out on all samples that have a reduced APCR or have a family history of factor V Leiden.

Prothrombin gene mutation (G-20210-A).

The mutation in the factor II gene (G-20210-A) is in the untranslated portion at start of the gene and is probably part of the the regulatory system for the gene. People carrying the mutation have higher levels of factor II than normal and the increased risk of thrombosis is thought to be a function of this.

Lupus anticoagulant (LA)

There is one acquired abnormality which is associated with an increased risk of venous thrombosis. That is the lupus anticoagulant, so called because it was first described in association with SLE. A lupus anticoagulant is an anti-phospholipid antibody.

Lupus anticoagulant (LA) results in prolongation of coagulation tests dependent on phospholipid e.g.APTT or DRVVT without specifically inactivating any individual coagulation factor. It is associated with a range of autoimmune disorders, infections and treatment with some drugs.

The presence of LA may be suggested by:

  1. Unexplained prolongation of APTT 
  2. Recurrent early foetal loss thought to be due to placental infarct
  3. Unexplained thrombotic tendency

These clots as in 2 and 3) are caused by platelet activation in the microcirculation

Other tests (second line investigations).

  • There are other gene mutations that are occasionally measured such as Methyl tetra hydorfolate reductase (MTHFR) and Thrombomodulin mutations.
  • The measurement of homocysteine is also occasionally measured. This is a functional measure of the MTFHR mutation.
  • Occasionally Individual clotting factors are measured such as FVIII, FVII and Fibrinogen. As inherited high levels of these factors can lead to a increased risk of thrombosis.

Who Should have Thrombophilia Screening?

Thrombophilia screening is expensive and time consuming and it is therefore important that it is targeted at the right people. The following guidelines should identify those individuals most at risk

  1. Patients with a known family history of any of the inherited thrombophilia factors.
  2. Patients with a family history of proven venous thrombo-embolism.
  3. Patients who have developed a thrombosis with no obvious precipitating cause or at a relatively young age. Presently this is advised below the age of 50.
  4. Women with a history of recurrent miscarriages should be screened for the lupus anticoagulant.

When should Thrombophilia Screening be Done?

Thrombophilia Screening should be done as soon as any of the risk factors above are identified. It is particularly important to screen before a patient is exposed to a known precipitating factor such as pregnancy or the oral contraceptive.

Thrombophila screening should not be done during the acute phase after the patient presents with a clot. Patients should be tested after the acute event and after any anticoagulation therapy (1 month post warfarin therapy).

Thrombophilia screens will not be processed on patients on mucosal heparin, as heparin interferes with all of the coagulation based assays of a thrombophilia screen and can cause reduced Antithrombin. Patients on low molecular weight heparin may be tested dependant on the coagulation screen results.

If it is not possible to carry out the thrombophilia screen after warfarin therapy or when patients are on lifelong warfarin a limited number of tests are available but it is important to inform the laboratory of the patients therapy. During warfarin therapy it is not possible to interpret Protein C and S results as these are both vitamin K dependant proteins and are reduced during the treatment.

The testing of children is not normally carried out. Any possible requirement to test a child for thrombotic tendency should be discussed with a Consultant paediatric Haematologist. (Dr Mike Richards 0113 20-65649).

What tests should be done?

It is recommended that a full thrombophilia screen be done on patients who fall into the at risk categories already defined. This consists of :

  • Protein C
  • Protein S
  • Antithrombin
  • APCR and Factor V Leiden
  • Protein gene mutation 
  • Lupus anticoagulant

Sample requirements:  2 * 3.2mL citrate (blue-capped) tube, 5mls of blood in EDTA (lilac capped) tubes, 1 * EDTA sample tube (lilac-capped), 1 * Clotted sample tube no anticoagulant (mustard-capped). NB Citrate sample tubes must be filled to the correct level.

Test

Reference Range

Anti-thrombin

0.8 – 1.2 IU/ml

Protein C

0.7 – 1.4 IU/ml

Protein S

Free - 0.7 - 1.25 IU/mL

APC resistance ratio

Ratios of greater than 2.5

Factor V Leiden

Reported as normal, abnormal (heterozygous) or abnormal (homozygous)

Prothrombin gene mutation Reported as normal, abnormal (heterozygous) or abnormal (homozygous)

The lupus anticoagulant

depends upon analysis of a series of clotting assays and would always be reported in an interpretive manner

Tests are performed by the department of Specialist Laboratory Medicine.


Page updated: 15/05/13 | Updated by: Michael O'Sullivan