United States standard diphtheria toxin for the Schick text and the erythema potency assay for the Schick text dose.

A diphtheria toxin shown to have high toxicity and avidity and to combine with antitoxin in multiple proportions was selected for the U.S. standard diphtheria toxin for the Schick test and freeze-dried. Assayed values per vial were 1.09 L(f), 1.09 L(r), 1,090 Schick test doses (STD), 33 LD(80), 38 LD(50), and 43,000 minimum skin reactive doses. One STD (L(r)/1000) is slightly more toxic than one unit of the international standard (L(f)/1,000). Experiments showed that the potency assay of Schick test toxin by guinea pig erythema toxicity, determined relative to the toxicity of the standard, was highly reproducible and significantly more reproducible than the lethal (minimum lethal dose) test and that the STD, defined as one L(r)/1000, was equivalent to approximately 1/50 minimum lethal dose. The erythema potency assay was prescribed in the U.S. standards for Schick test toxin effective in 1969.