LWMH is monitored using a chromogenic heparin anti-Xa assay, which measures the ability of heparin to inhibit the activity of a known amount of activated factor X (FXa) in the reagent. The reagent includes an excess of antithrombin so that heparin in the sample is rate-limiting reagent for Xa inhibition. Heparin in the patient sample inhibits the enzymatic conversion of a Xa-specific chromogenic substrate to colored product by factor Xa (residual Xa is assayed using a chromogenic substrate specific for Factor Xa). Standard curves are created using multiple concentrations of UFH and LMWH and are used to calculate concentration in the patient plasma. Samples are obtained 4 hours following subcutaneous injection of LMWH. Therapeutic levels for enoxaparin (Lovenox) are typically 0.60-1.00 IU/mL for twice daily dosing.