Mixing the patient’s IgG with thrombin-modified factor XIII (rA₂′) generates transamidase activity even in the absence of Ca²⁺. Recombinant factor XIII A₂ subunits (25 μg/mL rA₂ in 50 mmol/L Tris-HCl, pH 7.5) were treated with 1.25 U/mL of human -thrombin at room temperature for 20 minutes. Reactions were performed at 37°C in a total volume of 125 μL with 40 μg of normal (○) or varying concentrations of patient (•) IgG (isolated on a Protein A column from serum drawn 9/12/91), 50 mmol/L Tris-HCl, pH 7.5, 2 mg/mL N,N-dimethylcasein, 0.5 mmol/L dansylcadaverine, and 0.25 μg rA₂′. The increase in fluorescence intensity accompanying the amine incorporation reaction at 60 minutes is shown.