A thrombophilia screen should be requested following a second venous thrombosis,in young (<45yrs) patients in the absence of predisposing cause,those with a strong family history of thrombosis or proven thrombophilic defect, a history of recurrent foetal loss or thrombosis at unusual sites.
The thrombophilia screen consists of PT, APTT, Clauss Fibrinogen, Thrombin time Protein C & S, Antithrombin, Lupus Anticoagulant, Cardiolipin antibodies (see Immunology) and the screen for Factor V Leiden -Activated Protein C Resistance. A low ratio Activated Protein C Resistance will automatically generate a request to confirm presence of Factor V Leiden by PCR. (Prothrombin Gene Variant will be measured at the same time).Send 3 - 4 citrated (blue top) and a clotted (red top). Clearly indicate Thrombophilia Screen and Cardiolipin on the request form.
Thrombophilia test results may give misleading results in acute thrombotic situations or pregnancy. Do not request if the patient is on oral anticoagulants (Proteins C & S will be reduced and mild Lupus Anticoagulants can be missed).
Lupus Anticoagulant may be positive due to infection. All positive results should be repeated after 6 weeks to confirm.
Do not request Lupus Anticoagulant while patient on unfractionated heparin.
The negative predictive value of the D-Dimer assay for exclusion of PE/DVT is invalid for patients who have pre - existing conditions associated with raised D-Dimer levels e.g. post operation, malignancy, infection, late pregnancy etc.
Patients in a high risk group where PE/DVT is suspected, should proceed straight to the appropriate diagnostic test e.g. doppler, VQ scan.
Bleeding Time requests should initially be discussed with a Consultant Haematologist