Combined determination of Coxiella burnetii-specific immunoglobulin M (IgM) and IgA improves specificity in the diagnosis of acute Q fever.

Immunoglobulin M (IgM) and IgA responses in patients with acute Q fever were compared by enzyme-linked immunosorbent assay. An increase in both IgM and IgA was observed in paired sera from all 19 patients with acute Q fever, and both IgM and IgA levels showed good correlation with complement fixation test titers. Paired sera from 23 patients with infections other than Q fever were also tested. IgM levels were elevated in three of these patients, while IgA levels were elevated in three different patients (87% specificity for either IgM or IgA). As no patients in the disease control group showed elevated levels of both IgM and IgA, definition of a positive result as elevated levels of both IgM and IgA improved specificity to 100% without a decrease in sensitivity. This study indicates that detection of specific IgA is a useful adjunct to that of IgM in the diagnosis of acute Q fever.