Serodiagnosis of whooping cough in Belgium: results of the National Reference Centre for Bordetella pertussis anno 2013

Objective: Report on the pitfalls of serodiagnosis of pertussis in Belgium for 2013 by the NRC Bordetella. Methods: Determine cases of acute infection using an anti-pertussis toxin (PT) IgG antibody ELISA. Results: A total of 2471 serum samples were received. Clinical information on the duration of cough (at moment of blood sampling) is essential for a reliable interpretation of the results. In order to avoid false negative results, 213 samples for which this information was lacking were not tested. For a total of 2179 patients tested, 520 (23.9%) had antibody levels indicative of an acute infection, 261 (12%) samples were diagnosed as positive (indicative of a pertussis infection or vaccination during the last year), 143 (6.7%) samples were classified as doubtful and 752 (34,5%) were diagnosed as negative. The serodiagnosis of pertussis has limited value for the early diagnosis of the disease and PCR analysis on nasopharyngeal swabs is the method of choice during the first 2 weeks and always for young children < 1 year old. For sera collected during the first 2 weeks with anti-PT levels below the threshold for acute infection, a second sample collected 2-3 weeks later is needed a definitive diagnosis. For 503 (23.0%) early samples, a second serum sample was requested but not provided. For 85 patients, for whom a second sample was received, 12.9% were eventually diagnosed as having an acute infection. Conclusion: In order to generate reliable serodiagnostic results for pertussis, serum samples should preferentially be collected 3 weeks after onset of symptoms.