Solid-phase enzyme immunoassay for rotavirus antigen: faecal protease activity as a reason for false-negative results

Rabbits and guinea pigs were immunized with purified bovine rotavirus. Immunoglobulin G fractions of the resulting antisera were used in a standard four-layer solid-phase enzyme immunoassay (EIA) for rotavirus antigen in human faecal specimens. Samples negative for rotavirus in electron microscopy, when diluted in standard EIA buffers, regularly gave absorbance values lower than those obtained with buffer blank only. By further diluting of the samples the resulting absorbance values were found to increase to the blank levels. When all dilution buffers were supplemented with 1-5% of bovine serum, negative samples at any dilution gave absorbance values close to those of the buffer blanks. Similar results were obtained if the serum was replaced by 1-5 mM of phenylmethylsulphonyl fluoride, a synthetic broad spectrum serine-type protease inhibitor. Aprotinin, another protease inhibitor, was without effect. A similar inhibition pattern was obtained when faecal specimens were tested in a caseinolytic quantitative protease assay in the presence of the above inhibitors. These observations suggest that protease activity present in human faecal samples may cause false-negative results in solid-phase immunoassay for viral antigens, unless appropriate means are used to avoid this interference.