Lassa virus antigen levels in LF patients. Serum levels of LASV NP antigen in LF patients were quantitated by ELISA, using a sensitive caprine polyclonal antibody capture and detection sandwich method [11, 12]. Follow-up convalescent patients primarily displayed undetectable levels of LASV antigen. Viral antigen levels between nonfatal (LF NF) and fatal LF cases (LF F) were significantly different at the time of admission or testing (N = 87, p < 0.01). The average time from onset of symptoms to admission was 8.0 ± 3.7 days for LF NF patients, and 10.4 ± 4.5 days for LF F patients. The difference between these times was not statistically significant (p = 0.07).