Pitfalls of molecular diagnostic testing for Coxiella burnetii DNA on throat swabs

Coxiella burnetii, the causative pathogen of Q fever, is regularly detected in throat swabs from patients without serological evidence of Q fever infection. C. burnetii is also frequently found in bulk tank milk from dairy cows. We evaluated the false positivity rate of polymerase chain reaction (PCR) for C. burnetii DNA on throat swabs and investigated whether recent consumption of C. burnetii DNA-positive cow milk could contribute to this phenomenon.

C. burnetii PCR was performed on throat swabs obtained from patients in whom a throat swab was ordered for other diagnostic purposes; patients with community-acquired pneumonia (CAP); and healthy volunteers after consumption of commercial C. burnetii-containing cow milk products.

C. burnetii DNA was found in 5.0% of throat swabs ordered for other diagnostic purposes and in 15.3% of throat swabs from CAP patients without serological evidence of Q fever pneumonia. The positive and negative predictive value of C. burnetii PCR on throat swabs for Q fever pneumonia were 66.7% (95% CI, 38.0-88.2) and 48.9% (95% CI, 41.3-54.6), respectively. After consumption of commercial C. burnetii-containing cow milk products, C. burnetii DNA could be detected in throat swabs for as long as 30 min after ingestion.

C. burnetii PCR on throat swabs is of low diagnostic value for Q fever pneumonia and was false positive in 15.3% of CAP patients without Q fever pneumonia. Recent consumption of C. burnetii-containing products can influence the outcome of C. burnetii PCR on throat swabs. Therefore, diagnosis of C. burnetii infection should be made in combination with serology or PCR performed on blood.