Performance of seven serological assays for diagnosing tularemia

Abstract

Background: Tularemia is a rare zoonotic disease caused by the Gram-negative bacterium Francisella tularensis. Serology is frequently the preferred diagnostic approach, because the pathogen is highly infectious and difficult to cultivate. The aim of this retrospective study was to determine the diagnostic accuracy of tularemia specific tests.

Methods: The Serazym®Anti-Francisella tularensis ELISA, Serion ELISA classic Francisella tularensis IgG/IgM, an in-house ELISA, the VIRapid® Tularemia immunochromatographic test, an in-house antigen microarray, and a Western Blot (WB) assay were evaluated. The diagnosis tularemia was established using a standard micro-agglutination assay. In total, 135 sera from a series of 110 consecutive tularemia patients were tested.

Results: The diagnostic sensitivity and diagnostic specificity of the tests were VIRapid (97.0% and 84.0%), Serion IgG (96.3% and 96.8%), Serion IgM (94.8% and 96.8%), Serazym (97.0% and 91.5%), in-house ELISA (95.6% and 76.6%), WB (93.3% and 83.0%), microarray (91.1% and 97.9%).

Conclusions: The diagnostic value of the commercial assays was proven, because the diagnostic accuracy was >90%. The diagnostic sensitivity of the in-house ELISA and the WB were acceptable, but the diagnostic accuracy was <90%. Interestingly, the antigen microarray test was very specific and had a very good positive predictive value.

Figure 1

Hand-held immunochromatographic test for the serological diagnosis of tularemia (VIRapid®; Vircell S.L., Santa Fé, Spain). A test is interpreted as positive, if the specific line and the control line are positive (A). The test is valid, but negative, if only the control line is visible (B).

Figure 2

Receiver operating characteristic (ROC) curves were used to determine the cut-off value for the in-house ELISA (MedCalc software version 13.0.4, Oostend, Belgium).