The most convenient way to estimate low molecular weight heparin (LMWH) concentration in the plasma is to determine its antifactor Xa activity. This is usually performed with specific chromogenic assays. Chronometric methods, easy to perform, have recently been introduced in clinical laboratories. This study provides evidence that these chronometric assays, are unsuitable for measuring the antifactor Xa activity in the plasma of patients receiving such heparins, because they are also sensitive to the residual antithrombin activity of LMWHs. The clearance of the antithrombin activity of a LMWH is higher than that of the antifactor Xa activity. Therefore the antifactor Xa/antifactor IIa ratio of an HBPM continuously increases after parenteral injection. It results in a significant under estimation of the antifactor Xa activity when assayed with a chronometric method because the in vitro antifactor Xa/antifactor IIa ratio of a given LMWH used to construct the calibration curve is lower than that observed ex vivo after its parenteral administration.
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