Published: Journal of Analytical Toxicology, Volume 23, Number 7, Nov/Dec 1999, pp.637-638

Interference of Diphenhydramine with the EMIT II Immunoassay for Propoxyphene

To the Editor:

In the present letter we report our observation of diphenhydramine cross-reactivity with the propoxyphene EMIT II immunoassay. Diphenhydramine is an old antihistaminic agent also used as sleeping aid in many countries. It acts as a competitive inhibitor at the histamine H1 receptor site. It is extensively metabolized in the liver, and only 1% of the parent drug is found in urine. Therapeutic serum concentrations range from 0.025 to 0.11 mg/L. In fatalities, up to 31 and 64 mg/L have been detected in blood and urine, respectively (1).

Figure 1. Molecular structures of diphenhydramine and d-propoxyphene.

d-Propoxyphene is an opioid narcotic analygesic. Therapeutic serum concentrations range from 0.1 to 0.75 mg/L. d-Propoxyphene is extensively metabolized too; only about 1% of the parent drug is eliminated unchanged in the urine (2). Diphenhydramine and d-propoxyphene show some structural similarities (two unsubstituted benzene rings and an ethyldimethylamine side chain) (Figure 1).

In our laboratory, urine specimens are routinely screened for propoxyphene by EMIT II in drugs-of-abuse testing and in emergency cases. All assays were run on a Cobas Mira Plus instrument. In one urine specimen, a propoxyphene immunoassay result of 0.26 mg/L was observed. This is only slightly under the cutoff value of 0.3 mg/L. The result could not be confirmed by the subsequent gas chromatographic–mass spectrometric analysis, but high amounts of diphenhydramine and diphenhydramine metabolites were found.

Because of the structural similarities, we suspected this drug of being responsible for false propoxyphene immunoassay result. Spiking a blank urine sample with diphenhydramine and running the propoxyphene EMIT II test confirmed this hypothesis. The results from serial tests obtained from three different blank urine specimens spiked at diphenhydramine concentrations ranging from 5 to 200 mg/L are presented in Table I. Each urine specimen was run 5 times, resulting in 15 assays for each concentration. The cross-reactivities are weak; they decrease from 1.44% at 5 mg/L to 0.22% at 200 mg/L.

Diphenhydramine and propoxyphene are mainly metabolized through N-demethylation, giving nor- and dinordiphenhydramine and norpropoxyphene, respectively. These metabolites may also present cross-reactivity. Although weak and decreasing exponentially with increasing concentrations, the cross-reactivity of diphenhydramine with the EMIT II immunoassay must be taken into account and may give false-positive propoxyphene results in the case of severe diphenhydramine intoxication.

Serge Schneider and Robert Wennig
Laboratoire National de Santé
Division de Toxicologie
Centre Universitaire de Luxembourg
165A, avenue de la Faïencerie
L-1511 Luxembourg

References

1. R.C. Baselt and R.H. Cravey. Disposition of Toxic Drugs and Chemicals in Man, 4th ed. Chemical Toxicology Institute, Foster City, CA, 1995,
   pp 264–267.
2. R.C. Baselt and R.H. Cravey. Disposition of Toxic Drugs and Chemicals in Man, 4th ed. Chemical Toxicology Institute, Foster City, CA, 1995,
   pp 657–661.

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