MRX Antithrombin (AT) is a liquid, functional, chromogenic assay based on the inhibition of FXa in the presence of heparin. The chromogenic AT assay measures the AT activity (functional AT) by a two-stage method as follows: An excess of FXa is added to the citrated plasma sample pre-diluted with saline and in the presence of heparin. The residual FXa cleaves the peptide linked to the chromogenic FXa substrate, which results in the release of free p-nitroaniline (pNA). The amount of released pNA is measured at 405 nm and the absorbance is inversely proportional to the AT activity in the patient plasma, expressed as IU/mL or %.