A new chronometric assay to determine plasma antifactor Xa activity which is insensitive to the antithrombin activity of low molecular weight heparins

Some commercially available chronometric assays are influenced by the residual antithrombin activity of low molecular weight heparins (LMWH) and they underestimate the ex vivo anti Xa activity. We have evaluated a new kit (Staclot-Heparin) highly specific for the anti Xa activity of LMWH. A comparison with the results given by a reference chromogenic method (Stachrom-Heparin) indicates a very good correlation between the 2 assays (r = 0.95, n = 59). This new assay is not influenced by vitamin K antagonist treatments. Clinical biologists now have the possibility of determining the anti Xa activity generated by LMWH easily and accurately, using a chronometric assay.