Heparin (Anti-Factor Xa) Assay

Under certain circumstances (e.g. extremes of body weight, renal failure), it is useful to determine the actual concentration of heparin or low molecular weight heparin (LMWH) in the patient's plasma.

The test is performed by adding a known quantiy of purified Factor Xa to the patient's plasma in the presence of excess Antithrombin. The added FXa is inactivated by the Antithrombin in proportion to the amount of heparin in the patient's sample. Residual FXa is measured with a specific chromogenic substrate.

It is important that the Hemostasis and Thrombosis Laboratory be given information about the type of heparin being used to treat the patient since the assay is calibrated with the same material (eg. unfractionated heparin, LMWH, danaparoid, fondiparinux). Even among the LMWHs, erroneous plasma levels may be obtained if the heparin in the patient's plasma is of a different type to that used to calibrate the assay.

The Hemostasis and Thrombosis Laboratory can offer heparin (anti-FXa)assays to laboratories that need to establish their therapeutic range for unfractionated heparin (typically 0.3 -.0.7 units/ml).