False assignment of familial antithrombin III deficiency with the von Kaulla assay

As a serum assay, the von Kaulla assay for antithrombin III has been criticized on the grounds that consumption of antithrombin III during in vitro blood clotting may lead to falsely low assay values. Use of the assay to screen a Utah pedigree for familial antithrombin III deficiency lends support to this criticism. In all, the assay was performed on 150 persons. Low values on the assay were corroborated by performing an alternative coagulation assay, an amidolytic assay, and an immunoassay. While all persons with historic evidence for a diagnosis of familial antithrombin III deficiency had low assay values, eight of 23 persons with low values had normal values on the other assays.