The Sensitivity and Specificity of Lassa Virus IgM by ELISA as Screening Tool at Early Phase of Lassa Fever Infection

ABSTRACT

"Background: Early diagnosis; prompt treatment; and disease containment are vital measures in the management of Lassa fever (LF); a lethal and contagious arenaviral hemorrhagic disease prevalent in West Africa. Lassa Virus (LAV)-specific Reverse Transcriptase Polymerase Chain Reaction (RT-PCR) test; the gold standard for diagnosis; is unavailable in most centers. Serologic detection of LAV IgM is a more accessible tool and this work was to investigate its adequacy as an early marker for LF. Patients and Methods: A prospective case-control study conducted July 2007-March 2011 in a tertiary referral health center in Nigeria. Blood samples for test and control were evaluated for Lassa specific antigens and IgM using RT-PCR (primers S36+ and LVS 339) and indirect ELISA (Lassa Nucleo-protein (NP)-Antigen) respectively. RT-PCR outcome was used as standard to test for the sensitivity and specificity of IgM. Results: Of the 37 confirmed cases of LF infection by RT-PCR; 21 (57) were IgM positive. Amongst the 35 confirmed negative cases (control group); eight were IgM positive. The diagnostic sensitivity and specificity of the IgM assay were 57 and 77 respectively. The negative and positive predictive values of the IgM serological assay were 63 and 72 ; respectively; while the efficiency of the test was 67. Conclusion: The specificity and sensitivity of IgM as a screening tool for early detection of LF appear weak and; hence; the need for a reliable LF ""rapid screening kit"" since RT-PCR is unavailable in most centers. In the interim; ""high clinical index of suspicion;"" irrespective of IgM status; requires urgent referral to confirmatory centers."