The anti-factor Xa assay is designed to measure plasma low-molecular-weight heparin (LMWH) levels and to monitor heparin therapy. Notably, the results of anti-factor Xa testing cannot be used in the same way as the International Normalized Ratio (INR) is used to guide cumarin treatment: dose adjustments to remain in the therapeutic range have not been shown to be associated with better outcomes of care and lower rates of thrombotic and/or bleeding complications. The anti-factor Xa activity should therefore only be assessed in cases of considerable concern regarding an LWMH under- or overdosis. It is reasonable to lower the LMWH dose in case of a supratherapeutic anti-factor Xa activity. Increasing the LMWH dose in case of a subtherapeutic anti-factor Xa activity should however only be considered in case of morbid obesity or in the setting of breakthrough thrombosis in patients receiving therapeutic dosed LMWH, as long as the anti-factor Xa activity remains below the upper-level of the recommend range.