Immunoassay Evaluated For Tularemia Diagnosis

The laboratory diagnosis of tularemia is usually based on direct detection of the bacteria either by culture or by nucleic acid amplification techniques and on detection of elevated antibody titers.

A simple, rapid, and affordable point-of-care immunochromatographic assay has been evaluated that may assist in narrowing the differential diagnosis and would be useful for early diagnosis and treatment and for public health surveillance.

Scientists at Public Health Institution (Ankara, Turkey) tested a panel of 221 sera from 109 cases of tularemia as well as 236 sera from normal individuals or individuals with other infectious or autoimmune diseases. A commercially available immunochromatographic assay (ICA) for the serologic diagnosis of tularemia was evaluated, and the performance was compared with that of the current standard, the microagglutination test (MAT).

The VIRapid Tularemia ICA (Vircell; Granada, Spain) was manufactured using lipopolysaccharides (LPS) extracted by the hot phenol–water method from Francisella tularensis cell suspension. LPS was adsorbed on both the conjugate and the test line to generate a lateral flow ICA. A control line was also included to check the correct performance of the test. The test was performed by the addition of serum to the sample pad of the assay device followed by the addition of running fluid. Test results were read after 15 minutes by visual inspection for staining of the antigen and control lines. The intensity of staining of the reaction band was read visually and scored from one to three with a color reference diagram.

The ICA demonstrated 91.5% agreement with the MAT reference method and gave an overall sensitivity of 99.3% and a specificity of 94.6%. When only acute-phase sera were considered, the sensitivity and specificity of the ICA were 100% and 95.5%, respectively. No cross-reactivity was observed in the ICA with serum samples from normal individuals and patients with autoimmune diseases and bacterial, viral, and parasitic infections, although 4 out of 50 patients with brucellosis demonstrated positive results in the ICA.

The authors concluded that ICA showed higher positivity with early acute-phase sera than the reference MAT, while retaining high specificity. The assay should greatly improve the rapid detection of tularemia in the field and facilitate the rapid determination of the extent of public health threat especially in outbreak situations. Tularemia is an infection caused by F. tularensis, a Gram-negative, nonmotile coccobacillus, with a worldwide distribution in the northern hemisphere and diverse clinical manifestations. Humans become infected through arthropod bites, handling of infected animal tissues, or ingestion of contaminated water or food, and inhalation of infectious aerosols. The study was published in the September 2012 issue of the journal Diagnostic Microbiology and Infectious Disease.

Related Links:
Public Health Institution of Turkey
Vircell