Problem solved: a modified enzyme-linked immunosorbent assay for detection of human antibodies to pertussis toxin eliminates false-positive results occurring at analysis of heat-treated sera.

Enzyme-linked immunosorbent assay (ELISA) for measurement of antibodies to pertussis toxin (PT) is a widely used method for diagnosis of whooping cough and is also frequently used for seroprevalence studies and for assessment of antibodies after vaccinations against whooping cough. The recommended ELISA procedure for assessment of PT antibodies in human serum does, however, have a serious problem with false-positive results when heat-treated sera are analyzed. Historic sera might have been exposed to routine heat treatment, and diagnostic sera from warm geographic regions are at risk of unintentional exposure to heat. A modified version of the PT ELISA, incorporating a blocking step with 1% milk and addition of 0.1% milk to the dilution buffer, eliminates the false-positive phenomenon occurring with heat-treated sera. Results for serum antibody concentrations correlate well with the currently recommended method. This ELISA modification is straightforward and cheap, and it should be recommended at all analyses incorporating sera with unknown history of heat exposure.