International Standard for Antistreptolysin-O

A pool of highly potent freeze-dried sera from three patients with streptococcal infection has been examined by twelve laboratories in eleven countries and has now been established as the International Standard for Antistreptolysin-O by the WHO Expert Committee on Biological Standardization. The main part of this collaborative study consisted in a comparison of the proposed international standard serum with other standard preparations. Samples of serum from eight different patients were titrated by the different laboratories. The laboratories each carried out two series of tests, one using their own streptolysin-O preparations and the other using a particular streptolysin-O preparation supplied to all the participants. There was fairly good agreement between the results from the majority of the laboratories, particularly in the tests with the same preparation of streptolysin-O. Some of the variations could be traced to the use of unsuitable streptolysin reagents. On the basis of the results, the International Unit of Antistreptolysin-O has been defined as the activity contained in 0.0213 mg of the International Standard.