Early diagnostic performance of real-time PCR versus serology for murine typhus and Q fever in a public health setting

To assess the diagnostic performance of real-time PCR compared with conventional serology for murine typhus (MT) and Q fever (QF) in patients with undifferentiated febrile illness (UFI) in an endemic public health setting. This secondary analysis was nested within a prospective cross-sectional cohort of UFI conducted in the Canary Islands Health Service (2019–2022). Of 146 eligible patients, 78 fulfilled predefined analytical criteria and were included (39 evaluable for QF and 39 for MT). Real-time PCR was performed 5–10 days after fever onset and prior to antibiotic administration. Serology was performed from day 10 onwards. Sensitivity, specificity, diagnostic timing, and concordance between methods were analysed. Real-time PCR demonstrated 100% sensitivity and high specificity (96.3% for QF; 75% for MT) under strictly controlled conditions. Median time to molecular diagnosis was 6–7 days, compared with 26–27 days for serology. In a small number of patients, PCR was positive despite negative or inconclusive early serology. Overall, both techniques identified comparable proportions of confirmed cases, although PCR provided earlier microbiological confirmation. Real-time PCR and serology demonstrated comparable diagnostic performance under strictly controlled conditions. However, PCR provided earlier microbiological confirmation, particularly when performed 5–10 days after fever onset and prior to antibiotic administration. These findings support a stage-adapted, complementary diagnostic strategy in endemic settings, whereby molecular testing may be especially useful in patients with early UFI, while serology remains essential at later stages. Larger prospective studies with standardised follow-up would help to further validate these findings.