The effect of changing diagnostic method from culture to PCR on the number of episodes of human campylobacteriosis in Denmark: a retrospective study (2015–2022)

ABSTRACT We investigated whether the introduction of polymerase chain reaction (PCR) to replace culture as the primary diagnostic method for Campylobacter species in the Danish Departments of Clinical Microbiology (DCMs) was associated with a systematic change in the number of reported episodes of human campylobacteriosis from 2015 to 2022. We described a hypothetical scenario where PCR was solely used from 2018 to 2021. We analyzed the effect of primary diagnostic methods (culture or PCR) on the number of weekly episodes of human campylobacteriosis in a negative binomial regression adjusting for time, seasonality, COVID-19 restrictions, and DCMs. Furthermore, we applied the estimated PCR effect to the number of episodes that was identified using culture. Overall, PCR was associated with a 43% estimated increase of weekly campylobacteriosis episodes [rate ratio (RR) 1.43, 95% confidence interval (95% CI) 1.34–1.53]. Up to 17%–25% additional episodes would have been reported yearly from 2018 to 2021, had PCR been solely used. Through the lens of laboratory diagnostic methods, we described a systematic change in the number of reported episodes of human campylobacteriosis in Denmark. This is operationally relevant as considerably more episodes would have been identified had PCR been solely used. Changes in diagnostic methods should be considered in the epidemiological analysis of historical data. IMPORTANCE This study is important because it shows the potential epidemiological silence associated with the use of culture as the primary diagnostic method for the laboratory identification of human campylobacteriosis. Also, we show how polymerase chain reaction methods are associated with a systematic increase in the number of human campylobacteriosis episodes as reported by routine disease surveillance. These findings are operationally relevant and have public health implications because they tell how crucial it is to consider changes in diagnostic methods, e.g., in the epidemiological analysis of historical data and in the interpretation of future data in light of the past. We also believe that this study highlights how the synergy between microbiology and epidemiology is essential for disease surveillance.