Rapid screening of fentanyl (China White) powder samples by solid-phase radioimmunoassay.

We report the use of a solid-phase radioimmunoassay (RIA) to rapidly screen powder samples for various illicit fentanyl analogs. A 10-mg sample is dissolved in water, serial dilutions are made over a range of 1:10 to 1:100,000, and then duplicate 50-microL aliquots are analyzed by RIA. This method was verified by analyzing 18 "China White" samples, first by RIA and then by gas chromatography/mass spectrometry (GC/MS) with the spectrometrist blind to the results of the RIA. Of the samples, 11 were positive by radioimmunoassay and all were confirmed by GC/MS and found to contain fentanyl, alpha-methylfentanyl, fluorofentanyl, cis- and trans-3-methylfentanyl, thienylfentanyl, or cis- and trans-3-methylthienylfentanyl. The seven samples that were negative by RIA contained a variety of substances typically found in street drug samples--caffeine, codeine, and heroin--but none of the fentanyls. Differences in fentanyl concentration between samples were quite large, up to 300-fold; however, the drug seems to be uniformly distributed within a single sample. Thus, accidental overdose is more likely to result from the large intersample variability in drug concentration rather than the "clumping" of fentanyl within a sample. This solid-phase RIA provides a simple, rapid, and reliable screening method for detecting fentanyl and its illicit analogs in powder samples.