Improved ELISA for the detection of HBsAg

A sandwich ELISA for hepatitis B virus surface antigen (HBsAg) was developed using monoclonal anti-HBs for the solid phase and horse-radish peroxidase labelled sheep anti-HBs. The sensitivity was approx. 0.3 U/ml HBsAg, in the standard test procedure, comprising two incubation steps of 1 h at 37 degrees C, or in a shortened procedure comprising two incubation steps of 30 min at 50 degrees C. A slightly reduced sensitivity (approx. 0.5 U/ml) was obtained when the two incubations were combined in a one-step incubation for 1 h at 37 degrees C. All three procedures were completed by an incubation for 30 min at room temperature with peroxide and tetra-methylbenzidine. The number of false positives obtained when donor sera were screened was below 0.5%. False positive reactions occurred more frequently, but still to a limited extent, when previously selected sera containing rheumatoid factor or other possibly interfering factors were tested with the standard procedure. Most sera containing factors that interfere with a commercial ELISA for HBsAg using sheep anti-HBs coated plates, were negative. Rheumatoid factor positive sera seldom gave false positive results. The lower detection limit became approx. 0.1 U/ml when the cut-off was reduced, while the number of false positives in a donor population only increased to 1.5%. The results obtained with reagents from four different batches indicate a stability of up to 4 wk at 37 degrees C and for at least 26 wk at 4 degrees C.