The International Standard for Human Syphilitic Serum

Following the establishment by WHO of International Reference Preparations of Cardiolipin and Egg Lecithin to facilitate reproduction of cardiolipin antigens for use in serological tests for syphilis, a series of studies has been carried out between co-operating laboratories in several parts of the world, aiming at the establishment of an international standard preparation for freeze-dried human syphilitic serum.The WHO Expert Committee on Biological Standardization, in 1957, established the new International Standard for Human Syphilitic Serum which is now available to national laboratories from the International Laboratory for Biological Standards, Statens Serum-institut, Copenhagen. The International Unit of Human Syphilitic Serum is equivalent to 3.617 mg of the freeze-dried International Standard. A solution containing 8 International Units per ml can be obtained by reconstituting the contents of one ampoule of the International Standard in 6.1 ml of 0.6% saline.The new Standard will allow major serological laboratories to calibrate their national reference sera in terms of the International Unit; the comparability of results from different areas will be improved by this procedure.Technical details are given on the investigations preceding the establishment of the Standard. The analysis of the collaborative assay between co-operating laboratories also includes data on titres, reactivity, turbidity and other technical aspects.