A substrate amplification system for enzyme-linked immunoassays. II. Demonstration of its applicability for measuring anti-DNA antibodies.

We have recently described a substrate amplification system, based on the method of Self, which increases the sensitivity of alkaline phosphatase (AP)-dependent enzyme-linked immunosorbent assays (ELISA) by a factor of 30-50. This increase is achieved by having AP, the primary enzyme, produce an activator for a secondary enzyme-substrate system, within which marked amplification occurs. We have now demonstrated that this amplification method can be applied to the measurement of human antibodies to DNA. The sensitivity is greater by a factor of 10 than the conventional method, which uses p-nitrophenyl phosphate (p-NPP) as substrate. On replicate assays the method is reproducible, with a coefficient of variation of less than 0.1. This great increase in sensitivity should be of value in conserving specimens of serum and in screening monoclonal antibodies.