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)

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.