Factor X is a disulfide linked two-chain glycoprotein zymogen and is the precursor of the coagulation enzyme Factor Xa. Factor X is activated by Factor IXa in complex with Factor VIII, calcium and phospholipids during the intrinsic pathway and by Factor VIIa in complex with Tissue Factor, calcium and phospholipids during the extrinsic pathway of the coagulation cascade. The sensitive quantitative measurement of total human Factor X antigen in plasma samples is easily performed with this 96 well strip format ELISA kit. Factor X, Xa, and Xa in complex with inhibitors or cofactors will be detected by the assay. The concentration of Factor X in normal human plasma is 7-8 ug/ml. The assay measures total human Factor X in the 0.1-50 ng/ml range. Samples giving human Factor X levels above 50 ng/ml should be diluted in blocking buffer before use. A 1:1,000 dilution for plasma is suggested for best results. Human Factor X will bind to the capture antibody coated on the microtiter plate. After appropriate washing steps, anti-human Factor X primary antibody binds to the captured protein. Excess primary antibody is washed away and bound antibody is reacted with the secondary antibody conjugated to HRP. Following an additional washing step, TMB substrate is used for color development at 450nm. Color development is proportional to the concentration of Factor X in the samples. A standard calibration curve is prepared using dilutions of purified Factor X and is measured along with the test samples. All reagents and standards are provided in these ELISA kits.