The assay is performed as a direct ELISA to detect LASV NP antigen. Diluted samples, reference, and controls are incubated in microwells coated with purified LASV NP specific polyclonal antibody. Incubation allows the NP antigen present in the samples to react with the immobilized antibody. After the removal of unbound serum or plasma proteins by washing, purified LASV NP specific polyclonal antibody conjugated to horseradish peroxidase (HRP) is added to form complexes with the bound antigen. Following another washing step, the bound anti-LASV NP HRP conjugate is assayed by the addition of TMB substrate. Color develops in the wells at an intensity proportional to the concentration of LASV NP antigen. Optical Density (O.D.) results are obtained by reading the absorbance (A450nm minus A620nm) using an ELISA plate reader. NP antigen concentration can be estimated from the reference curve prepared from the NP Antigen Reference provided in the kit. It is recommended that the user establish a cut-off for the study population using normal (non-febrile) control samples.