Biological Assay of Lots of Histoplasmin and the Selection of a New Working Lot

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Abstract
A description of the procedures employed in developing a large new lot of histoplasmin (H-42) for use by the Division of Tuberculosis (USPHS). This project became , necessary when the histoplasmin (H-15) employed by the Division in most of its studies of histoplasmin sensitivity was nearly exhausted. Since there was no standard reference product, the potency of the new material was adjusted to match the old in order to achieve continuity in the research programs. This was done by determining what dilution of the new material would match the previously used 1:1000 dilution of H-15, arbitrarily called "the standard." Trial dilutions were compared to the standard by human skin testing in which the standard was placed in one arm and a trial dilution in the other. A sensitive measure of the relative potency of each trial dilution, called the critical difference, consisted of the difference of: (1) the percentage of persons whose reaction to the standard was the larger, and (2) the percentage of persons whose reaction to the trial dilution was the larger. Critical differences for each of 3 trial dilutions permitted estimation of a matching dilution associated with a zero critical difference. Four lots of histoplasmin were considered in the selection of the new product. Tests of each lot and certain combinations of them led to the final selection of & pooled lot (designated H-42) involving 3 of them. The estimated matching dilution for this pooled lot was 8:1000. Included in the report are the results of duplicate tests with H-15, as well as assays of other lots of histoplasmin, namely H-40 and a Lilly Co. product, against the standard.
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