Summary. Pernasal aspirate (PNA) was obtained from 543 children during a 6-month period when whooping cough was prevalent. Three tests for diagnosing pertussis were performed on the PNA : (a) examination of direct smears by immunofluorescence (IF) for Bordetella pertussis; (b) culture ; and (c) estimation of B. pertussis-specific immunoglobulin-A antibody (P-IgA) by an enzyme-linked immunosorbent assay (ELISA). On clinical review, 395 children were assessed to have had pertussis (P children) and 148 children not to have had pertussis (non-P children). The non-P children comprised 66 admitted to hospital for acute respiratory infections and 82 outpatients suspected of having pertussis. Analysis of the results of the tests on the PNAs of the non-P children helped to assess the P-IgA test. The analysis showed that artificial immunisation against pertussis did not affect the antibody results, but that non-specific positive results occur requiring the labelling of many P-IgA results as "doubtful". Among the 395 P children, 36% yielded positive cultures and more than half of these also had positive IF tests. The ELISA for P-IgA was positive in 24% of all the P children, equivalent to nearly 40% of the culture-negative P children. For the 148 non-P children, IF and culture-negative by definition, the P-IgA test was positive in 9%. The antibody test result was doubtful in 28% of the P children and in 40% of the non-P children. Estimation of P-IgA antibodies in PNA is a useful and economic complement to culture and IF in the diagnosis of pertussis. The occurrence of non-specific positive results makes evaluation of the clinical features of the child essential to the interpretation of the test.
IntroductionIn affluent countries, whooping cough is a rare cause of death. For this reason the frequency with which the infection causes prolonged family distress is liable to be discounted. Johnston et al. (1985) recently wrote : "Parents suffered especially from fears for the life and health of their child and from loss of sleep". In the absence of an effective way of shortening the course of the illness (Broomhall and Herxheimer, 1984), prevention continues to be the only practical option.