Evaluation an in–house IgM-ELISA for the diagnosis of human leptospirosis

Background: Leptospirosisis a very common zoonosis in the world. Culture is low sensitive with high rate false negative. So, serological assays are best alternative way for its diagnosis. Microscopic agglutination test (MAT) is gold standard but performing it requires a panel of some standard strains and need periodic subculturing of them, and also requires double sera with at least two weeks interval to investigate seroconversion. Furthermore, other serological methods should be investigated. The aim of this study was to evaluate an in-house IgM-ELISA developed by using antigen extracted from endemic isolates. Material and Method: I4 endemic isolates belonged to the serogroups: Icterohaemorrahgia, Pomona, Hardjo, and Gripotyphosa, were inoculated in EMJH to take well grown cultures. Whole antigen was extracted from each culture by Freezing-Thawing method in distilled water. Same amount of extraction of each culture with same OD number in 550nm were mixed together and were used for coating Elisa plates. Antihuman IgM conjugated with alkaline phosphatase were used in this assay. We used a commercial quantitative IgM-ELISA (SERION ELISA classic) for cut off determination. MAT was used for confirmation positive and negative cases. Sera with titer ≥ 1:100 in MAT and positive criteria in commercial quantitative IgM-ELISA were considered as positive cases. Results: 98 positive cases and 54 negative cases were chosen by screening 200 sera of patients suspected to leptospirosis by using MAT and commercial quantitative IgM-ELISA. We also used 30 sera of patients affected by hepatitis B, salmonelosis, and brucellosis as control cases. 88 of 98 positive cases were positive (false negative=10), 1 of 54 negative and all control case were negative (false positive =1) in the test. Sensitivity, specificity, PPV, NPV, and accuracy of the test were evaluated :99.0% , 89.1% , 90.75 , 98.8% , and 94.25 , respectively. Conclusions: ELISA for measuring specific IgM to leptospires antigen(s) could be a good alternative to MAT, which is not a routine diagnostic assay to perform in clinical diagnostic laboratories and only is reliable when there is paired sera. Sensitivity and specificity of the assay is dependent to several factors, especially to the type of antigen coated on plates, quality of assay materials, and also to the time of sampling. Sera of days ≥ 6 of the disease has enough antibodies to measure and a common antigen extracted from several common pathogenic leptospires, especially from endemic isolates, could be more helpful to increase accuracy of the assay