Sir.—While reading the article by Anderson et al,1 we noticed that the indirect hemagglutination (IHA) values were converted to international units (IUs) by using an equation that, we assume, based on their reference 24, was the one derived at the Institutes of Laboratories, Massachusetts Department of Public Health, Boston. Our experience with this equation may be important.
In the 1960s, while under contract to the National Communicable Disease Center, we studied the serologic responses to the antigens in the available diphtheria-tetanus-pertussis vaccine. For practical reasons, the IHA technique was also chosen to determine the response to both tetanus and diphtheria toxoids on all samples. The procedures for the IHA tests for both tetanus and diphtheria, as standardized at the Institutes of Laboratories, were followed using the Institutes' purified toxoids. Conversion of IHA titers to IUs was carried out using its equation. We sought confirmation mouse neutralization antibody levels