The evaluation of a radioimmunoassay for diphenylhydantoin using an iodinated tracer.

Radioimmunoassays (RIA) for diphenylhydantoin (DPH) are described using a rabbit antiserum to the immunogen, diphenylhydantoin-3-omega-valerate bovine serum albumin, and DPH labelled with carbon-14 (14C), tritium (3H) or iodine-125 (125I). The [125I]DPH label is prepared by iodination of DPH-3-N-ethyl-acetamido-tyrosine ethyl ester, and is stable for several months. The radioimmunoassays are rapid, precise, specific and sensitive requiring only 50 microliter serum for duplicate analyses. Serum DPH levels measured by the three RIA methods correlate well with each other (3H vs. 14C, r = 0.96 : 3H vs. 125I, r = 0.97) and with values obtained by a conventional gas chromatographic method (125I RIA vs. GC, r = 0.95). However, RIA employing the iodinated DPH tracer has significant practical advantages over the other methods and is well suited to large scale clinical application.