The Half-life of Factor XIII in Vivo

S ummary . Factor-XIII levels were measured in the plasma of two patients with congenital deficiency following infusions of fresh frozen plasma at times remote from an episode of bleeding. The disappearance curve of factor XIII from the plasma showed an initial rapid and then a slow component, the half-life of the slow phase being about 12 days which is considerably greater than most previous estimates. The therapeutic implications of the prolonged effect of infused factor XIII are discussed.