Thrombophilia Screen

General Information

Note: For external requests please click Here for Coagulation Disorders request form. Please complete all information fully. Samples for Factor V Leiden and/or Prothrombin G20210A should be sent directly to Heartlands Hospital (the assay is NOT performed at Queen Elizabeth Hospital). These should be refrigerated and then sent at ambient temperature packaged separately from any frozen samples (samples stored in this way are viable for up to 4 weeks).

Samples that meet the following criteria will not be processed:

• Haemolysed, clotted, underfilled or overfilled samples.
• Samples >12 hours old from the time of collection.
• Samples received from other hospital laboratories that have been frozen and thawed.
• Samples which fail the minimum data set.

Assay limitations:

• Antithrombin: not affected by Heparin (UF/LMWH) up to 4U/mL, alpha-1antitrypsin up to 4mg/mL, Heparin Cofactor II up to 4U/mL, Haemoglobin up to 500mg/dL, Bilirubin up to 40mg/dL and triglycerides up to 2300mg/dL.
• Protein C: not affected by Heparin (UF/LMWH) up to 2.0 U/mL, Haemoglobin up to 200mg/dL, Bilirubin up to 21mg/dL.
• Protein S: not affected by Heparin (UF/LMWH) up to 1.5 U/mL, Haemoglobin up to 200mg/dL, Bilirubin up to 25mg/dL.Lipaemic and icteric samples may also interfere with the analyser optical reading unit used in the colorimetric assay method.
• Lupus: There is no reliable method for testing for the presence of a lupus anticoagulant when the patient is receiving vitamin K antagonists and therefore LA testing is not recommended in patients receiving warfarin because exclusion of a LA is problematic whilst the international normalized ratio (INR) is in the therapeutic range.
• Lupus tests should not be performed if the patient is receiving therapeutic doses of unfractionated heparin, because this may cause erroneous results. Lupus inhibitor screening should be delayed until the heparin therapy has been stopped. Low dose subcutaneous unfractionated heparin and low molecular weight heparin have less effect on the assay and most commercial reagents contain a heparin neutralizing reagent sufficient to cover prophylactic doses.
• Mixing tests are a criterion for Lupus and improve the specificity. However, they introduce a dilution factor and may make weak Lupus samples appear negative. In the absence of any other causes of prolonged clotting times, such samples should be considered Lupus positive if the screen and confirmatory tests on undiluted plasma give positive results. Whenever possible, this should be confirmed by testing a fresh sample.

Notes

All requests for Thrombophilia Screening must be approved by a Haematology Consultant prior to sending samples to the laboratory. Please refer to Clinical guideline [NG158]

Please click Here for further information (see section 1.9 for relevant clinical indications).