A case report of false-negative Q fever serology demonstrating a “prozone” effect

ABSTRACT Background Q fever is often part of the differential diagnosis for fever in a returning traveler with relevant exposure history. Precise diagnosis is crucial for directing therapy and repeating serologies to ensure the patient is not at risk for chronic disease. As molecular testing is not always available in the clinical setting, and the sensitivity of such testing declines with the duration of illness, diagnosis often relies on convalescent serology. Testing within 2 weeks of symptom onset has led to false-negative testing results due to a lag in the development of an antibody response. Case Summary In this case of Q fever in a patient returning from Oman, we observed false-negative results from a commercially available Q fever immunofluorescent antibody test performed 11, 18, and 43 days after symptom onset at separate laboratories, which were corrected by dilution of the serum beyond what is typically performed on clinical specimens, suggesting a “prozone” effect in Q fever serology. Conclusion Clinicians and laboratories should be aware of this prozone effect leading to false-negative Q fever serology, especially when testing patients with a high pre-test probability of having Q fever.