Comparison of PCR and serology assays for early diagnosis of acute Q fever

Abstract

Q fever is a potentially severe disease which can occur in large outbreaks of acute infections and is a possible bioterrorism agent. In order to lessen the delay in diagnosing acute Q fever, we compared LightCycler Nested PCR (LCN-PCR), a rapid nested PCR assay that uses serum sampled early during the disease as a specimen and the LightCycler as a thermal cycler, to serology by indirect immunofluorescence. We used the 20-copy htpAB-associated element as the DNA target. The detection sensitivity of this method was one Coxiella burnetii DNA copy. We applied this method to the first serum samples taken from 100 patients diagnosed in our laboratory as having acute Q fever on the basis of clinical manifestations and serology and to 80 controls. The LCN-PCR had a specificity of 100%. The sensitivity was 26% when no antibodies were detected but only 5% with seropositive patients (P < 10(-2)). The technique was most efficient in the first 2 weeks following the onset of symptoms (P = 0.02), when its sensitivity was 24% compared with 14% for serology. With combined use of LCN-PCR and serology within the first 2 weeks, the sensitivity was significantly increased over that with serology alone (P < 10(-2)). Thus, we propose a strategy for improving the early diagnosis of acute Q fever where LCN-PCR should be performed together with serology in the first 2 weeks of the disease but should be reserved for seronegative patients in the next 2 weeks and not used later than 4 weeks following onset, when serology is highly sensitive.

FIG. 1.

Cumulative sensitivities of LCN-PCR and serology. Each white circle represents the ratio of sera exhibiting titers of positive IgM antibodies to phase II C. burnetii of >1:50 among all samples tested until a given week. Black squares represent the ratio of positive LCN-PCR sera among all samples tested until a given week. The bold line represents the trend line for LCN-PCR positivity, whereas the dashed line represents the trend line for sera exhibiting titers of IgM antibodies to phase II C. burnetii of >1:50. Curve equations and determination coefficients (R²) are indicated.

FIG. 2.

Diagnostic strategy for the early diagnosis of acute Q fever.