In attempts at standardization, a World Health Organization reference plasma with assigned pFXIII activity and FXIII-A2B2 antigen values in international units (the 1st international standard for factor XIII, plasma) was established.22 Neutralizing autoantibodies against FXIII-A can be detected in a mixing study; in the mixture of patient and normal plasma, the FXIII activity of normal plasma will be inhibited by a neutralizing autoantibody. The diagnosis of nonneutralizing inhibitors requires binding assays, that is, the detection of the binding of patients' immunoglobulin (IgG/IgM) to purified pFXIII and purified FXIII subunits in enzyme-linked immunosorbent assays or dot-blot arrangements. The demonstration of accelerated clearance of administered FXIII concentrate from the circulation also helps in making the diagnosis.