Factor antigen assays: FVIII, FIX, FXI

Coagulation factor deficiencies can be quantitative, involving a reduced concentration of a normally functioning molecule, or qualitative, involving dysfunctional molecules but typically normal concentration, or at least a higher concentration than that indicated from biological activity alone. Coagulation factor antigen assays are sandwich ELISAs that measure concentration irrespective of function. The coagulation factor being measured is captured by antibodies bound to microtitre plate wells, incubated, and unbound material then washed off. A solution of antibody to the coagulation factor that is conjugated to an enzyme is added to ‘tag’ onto any captured coagulation factor. Unbound conjugate is washed off and a substrate for the enzyme is added, the product of the enzyme-substrate reaction being coloured. Colour intensity is in direct proportion to the degree of conjugate-binding, itself proportional to the amount of coagulation factor capture and thus, concentration.