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Laboratory tests

The major aims of managing patients with blood clots is to initiate therapy to resolve the acute blood clot, then to determine what clinical factors (including immobility, smoking, oral contraceptives, pregnancy, hormone replacement therapy), inherited conditions and laboratory abnormalities may have contributed to the formation of the blood clot. Clinicians are then able to determine the long-term risk for the patient of recurrent blood clot formation. It should be noted that laboratory testing is only one component of this process and that interpreting these results and determining the need for further blood thinning may require referral to a haematology specialist.

Tests which may be undertaken to assist the haematologist in determining the risk of recurrent blood clots include: PT, APTT, FBC, activated protein C resistance (APCR), Factor V mutation (Leiden) assay (when APCR is abnormal), homocysteine, cardiolipin antibodies, protein C, protein S, antithrombin and a prothrombin 20210 mutation test. If the APTT testing is prolonged, lupus anticoagulant testing may be performed.

Testing may be affected by the type of anticoagulant medications that a patient is taking and underlying medical conditions including pregnancy. As such, testing may not be completed in one session and may require repeat testing when a patient is not taking anticoagulant medications.

Tests for hypercoagulable disorders
Test Measures Ordered When/To Abnormal Results May Indicate
Cardiolipin antibodies Presence of antibody Evaluate recurrent blood clots and/or miscarriages Antiphospholipid syndrome
Antithrombin activity  Activity of antithrombin Evaluate recurrent blood clots Low activity may increase thrombotic risk
Antithrombin antigen Quantity of antithrombin Activity is consistently low Decreased production or increased use of factor, may increase thrombotic risk
APCR (activated protein C resistance) Resistance to degradation of activated factor V by APC Evaluate recurrent blood clots Need to confirm by checking for Factor V mutation (Leiden)
D-dimer Level of a specific type of crosslinked fibrin degradation product Evaluate blood clot formation during bleeding and clotting episodes If elevated, indicates recent clotting activity. May be due to acute or chronic condition, such as a thromboembolism or DIC (disseminated intravascular coagulation)
Dilute Russell Viper venom test (dRVVT) Time to clot formation test, evaluates the common pathway of coagulation. Dilute refers to lipid concentration. Evaluate recurrent blood clots, when APTT is prolonged, looking for a lupus anticoagulant. When prolonged, suggests lupus anticoagulant may be present, increased risk of thrombosis.
Factor V mutation (Leiden) Genetic mutation that results in formation of an activated Factor V that resists degradation by APC Recurrent blood clots Increased risk of thrombosis
FDP (fibrin degradation products) Reflection of clotting and fibrinolytic (clot breakdown) activity Evaluate bleeding and clotting If increased, indicates recent blood clot formation and breakdown
Fibrinogen Amount of fibrinogen in the circulation. Evaluate bleeding and clotting If low, may indicate decreased production, increased use, or a functional defect. May be elevated with inflammation as, it is an acute phase reactant
Homocysteine Level in blood Recurrent blood clots If elevated, increased cardiac risk and risk of arterial thrombosis. Also elevated with vitamin B12 or folate deficiency
Lupus anticoagulant (LA) Panel of tests are used to check for lupus antibody Recurrent blood clots and/or miscarriages, prolonged APTT When APTT or LA sensitive APTT and dRVVT are prolonged it suggests LA, usually confirmed with additional testing; if present, increased risk of arterial and venous thrombosis
LA-sensitive APTT
Time to clot formation test When lupus anticoagulant (LA) suspected If prolonged and ‘corrects’ to normal when phospholipids added, may be due to LA
tetrahydrofolate reductase (MTHFR)
Genetic mutation Homocysteine level is elevated with no clear acquired aetiology. Increased risk for developing elevated homocysteine levels.
Platelet neutralisation procedure (PNP) Timed test using either the APTT or the dRVVT, using platelets as a source of phospholipids Evaluate prolonged APTT and recurrent blood clots If test corrects to normal with the addition of platelets, may indicate presence of a lupus anticoagulant
Protein C activity Function of Protein C Recurrent blood clots Protein C helps slow down the coagulation cascade by degrading activated Factors V and VIII. If activity is low, there is an increased risk of thrombosis
Protein C antigen Quantity of protein C When protein C activity is low If decreased, may be due to an inherited or acquired condition. Increased risk of thrombosis
Protein S activity Function of protein S Recurrent blood clots Protein S is a cofactor, helps protein C
Protein S antigen (free and total) Quantity of total and free protein S When protein S activity low Only free protein S is available to assist protein C; total protein S includes free protein S and protein S bound to C4b-binding protein.
Prothrombin 20210 mutation Genetic mutation Recurrent blood clots Increased risk of thrombosis
Prothrombin Time (PT) Time to clot formation As part of an initial workup for bleeding or clotting, monitor anticoagulant therapy Prolonged PT is suspicious for a coagulation factor deficiency and suggests the need for additional tests.
Warfarin therapy is monitored with the International Normalised Ratio (INR) which is a test based on the PT. The INR assists medical staff to determine how effective the warfarin is at preventing unwanted clotting.
APTT (actvated partial thromboplastin time) Time to clot formation test, Evaluates the intrinsic and common pathways of coagulation cascade Screens for lupus anticoagulant, monitor anticoagulant therapy Prolonged APTT suggests need for further tests. May indicate nonspecific inhibitor (such as lupus anticoagulant)

Last Review Date: November 1, 2019

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