Modification of the theophylline radioimmunoassay.

The routine utilization of a commercially available radioimmunoassay (RIA) for theophylline (GammaDab), although reliable, is currently prohibited by the high cost of the reagents. In an effort to reduce these costs we have diluted the [125I]theophylline tracer and theophylline antiserum reagents by one-half, contrary to the manufacturer's recommendations. We have demonstrated that an excellent correlation exists (r = 0.968) between our modified RIA method and a conventional high-performance liquid chromatographic technique, despite reagent dilution. Accordingly, our reagent costs have been reduced by half. We conclude that the GammaDab kit reagents can be diluted twofold and still provide an accurate determination of serum theophylline. We must also emphasize that any further alteration(s) of this theophylline RIA procedure would require a thorough evaluation before its routine use could be substantiated.