From 92,686 sera sent from hospitals throughout Japan to the Special Reference Laboratories, for CH50 assay, we were able to classify 80 patients as C9-deficient using a sensitive screening test, as well as hemolytic and immunochemical C9 assays. The incidence of C9 deficiency was determined to be 0.086%, and there were no distinct differences among the eight areas of Japan tested. Serum CH50 levels of these C9-deficient patients varied widely (9.4–63.8 U/ml), and exhibited a higher value (average: 34.1 U/ml) than that of healthy C9-deficient individuals, probably due to elevated C3, C4, and C5 levels. These patients suffered from a variety of autoimmune, renal, and infectious diseases, which, however, are thought to be only incidentally associated with C9 deficiency.

Standard criteria for the positivity of the IgG antibody to CL and PS and the prolongation of APTT in pregnancy should employ the values of normal pregnant women rather than those of non-pregnant women.