To highlight an analytical interference involving nefopam and its metabolites on urinary benzodiazepine screening with Enzyme Multiplied Immunoassay Technique (EMIT) Atellica CH from Siemens Healthineers, and to evaluate two other immunoassays: MULTIGENT on Architect from Abbott Diagnostics and Cloned Enzyme Donor Immunoassay (CEDIA) Thermo Fisher kits on Cobas from Roche Diagnostics.

An analytical interference was suspected for a 58-year-old-woman hospitalized for unexplained malaise. The urinary EMIT Atellica benzodiazepine immunoassay was positive. Screenings by liquid chromatography coupled with diode array detector (LC-DAD) and liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) identified nefopam and metabolites but no benzodiazepine. Drug-free urine has been spiked at different concentrations of nefopam (187,5–3000μg/L) and tested on EMIT Atellica assay. Three urines from nefopam-treated patients and two negative controls have been selected to assess this cross-reactivity. Nefopam quantification has been performed by LC-DAD. Different immunoassays were evaluated: EMIT on Atellica, MULTIGENT on Architect and CEDIA on Cobas.

Drug-free urines spiked with nefopam were negative on EMIT Atellica immunoassay. The interference observed was confirmed for nefopam-treated patients (2/3) on EMIT Architect. For one patient, an interference was observed with a low concentration of nefopam (< 5μg/L) but associated with the presence of two metabolites. All CEDIA immunoassay results were negative. This interference is not mentioned in supplier records.

Comparison of three immunoassay techniques revealed a heterogeneity of the interference. Nefopam metabolites may be responsible for this cross-reactivity, suggesting the importance of testing them during the kits evaluation phase. This study highlights the need to confirm immunoassay results by more specific techniques.