An evaluation of the use of a pH indicator for the detection of β-lactamase in enzyme immunoassay

Abstract The use of a mixed pH indicator containing bromocresol purple and bromothymol blue was evaluated for the detection of β-lactamase activity in enzyme immunoassays (EIA) based on this enzyme. The EIA was found to correlate well with a bioassay for antibodies to tetanus toxoid in sheep sera. Results could be read spectrophotometrically at 450 nm or visually as a colour change from dark blue to yellow. The mixed pH indicator was found to have a number of desirable features including good stability, ease of preparation and a sharp colour change which makes the system suitable for visual determination of titration end points.