A 58-year-old dairy farmer presents with progressive exertional dyspnoea. Echocardiography shows aortic valve vegetations with severe regurgitation. Three sets of blood cultures are sterile. He reports regular contact with parturient cows. The most appropriate diagnostic test for the likely etiology is:

• A Blood culture on buffered charcoal yeast extract agar
• B High-titre IgM antibodies against phase II antigen of Coxiella burnetii
• C Weil-Felix agglutination with Proteus OX-19 antigen
• D High-titre IgG antibodies against phase I antigen of Coxiella burnetii ✓

Explanation

Chronic Q fever most often presents as culture-negative endocarditis in patients with valvular disease and livestock exposure. Diagnosis rests on indirect immunofluorescence showing markedly elevated IgG against the phase I antigen, conventionally a titre above 1:800. Acute Q fever shows a phase II predominant response, and the shift toward phase I antigens reflects antigenic modification of the organism during prolonged intracellular persistence. Coxiella does not agglutinate Proteus antigens and does not grow on BCYE agar.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.