Antibodies to bovine serum albumin in human sera: problems and solutions with casein-based ELISA in the detection of natural Japanese encephalitis virus infections.

An ELISA system for measuring antibodies to nonstructural protein 1 (NS1) of Japanese encephalitis virus has already been established. This system uses an ELISA diluent containing casein, instead of bovine serum albumin (BSA). During a survey, we found that 21 (21%) of 102 children aged 1-5 years, who had no history of Japanese encephalitis vaccination and were without detectable neutralizing antibodies, showed positive results with this ELISA system. Western blotting analysis showed that sera from 19 (91%) of these 21 subjects had antibodies to BSA, but not NS1. These sera reacted with BSA antigen remaining in immunoaffinity-purified NS1 antigen. One solution to this problem was to reduce the BSA level to < or =1% of the NS1 amount. Another was to use a control well sensitized with BSA with the same amount as that contained in the NS1 antigen preparation.