Provided are compositions and methods for accurate determination of the concentration of anticoagulant in a sample such as a blood or plasma sample. The compositions can contain a Factor X compound and Factor V compound, and additional components as well. Methods for performing the assay include one-step methods in which a sample is added to a coagulation assay composition, and time is monitored from the point of adding the sample and coagulation assay composition to an endpoint, such as clot formation.