Factor XIII Assay by an Isotope Method

S ummary . The urea and acetic acid solubility tests to detect deficiency of factor XIII were compared. The urea test is more specific, but the acetic acid test is more sensitive, quicker, and therefore more convenient.
A radioisotope assay method for factor XIII (transamidase) is described. Incorporation of ¹⁴C putrescine into casein provides a substitute for the linkage by transamidation of fibrin monomers. Fresh plasma, fresh frozen plasma and out-dated bank plasma have similar transamidase activity. Slight instability is apparent after storage for 3 days at room temperature. The normal range of plasma factor XIII is wide. Aged serum contained 12.5% of normal plasma transamidase. Platelets are a rich source of transamidase activity, particularly after disruption. Approximately half the total whole blood transamidase resides in the platelets and the remainder in the plasma. Leucocytes and red cells contain no transamidase. After lysis, however, red cells do reveal activity in this assay system. This cross reaction requires further elucidation.
The venom ('Reptilase') of the Brazilian viper, Bothrops jararaca , can activate factor XIII in similar fashion to thrombin.