A variety of substrates can be employed in enzyme immunoassays (EIAs) for the measurement of enzyme-labeled immunoreactants. We compared the sensitivities of a fluorescent and a colorigenic substrate in an EIA system for the measurement of Haemophilus influenza purified polyribose phosphate. After a 10-min substrate incubation, the EIA in which the fluorescent substrate was used could detect 10 pg of polyribose phosphate per ml, whereas the EIA in which the colorigenic substrate was used required the addition of 640 pg of polyribose phosphate per ml to generate a positive reading. However, the use of longer substrate incubation periods led to an increase in sensitivity of the colorigenic EIA. After an incubation period of 240 min, the sensitivity was equal to that of the EIA in which the fluorescent substrate was used. These results suggest that the ultimate limit of sensitivity of EIA systems is determined by the nature of the antigen-antibody reactions. However, the use of high-energy substrates in EIA systems can allow for the attainment of maximal sensitivity after short enzyme-substrate incubation periods.
