Increased False Positives vs Increased Interference Flags: Comparison of 2 Ethyl Glucuronide Immunoassays

Screening for alcohol use in a population to treat pain and substance use disorders is important for monitoring adherence to treatment plans. Urine screening for ethanol exposure by immunoassay can provide a cost-effective testing option, with a faster turnaround time to result in comparison to LC-MS/MS. The purpose of this study was to compare two ethyl glucuronide (EtG) immunoassays to determine which method would be better suited for our laboratory work flow and patient population.

Two EtG immunoassays were evaluated to assess false-positive results and interference flags for alcohol biomarker testing. The EtG enzyme immunoassays were verified and put into use for patient testing during 2 separate timeframes.

Both assays compared qualitatively well to LC-MS/MS for positive EtG (>500 ng/mL) patient samples. In our patient population, the ARK EtG assay had a presumptive positivity rate of 13.81%, a false-positive rate of 1.35%, and an interference rate of 0.45%. The Immunalysis assay had a 9.87% presumptive positivity rate, a false-positive rate of 0.15%, and a 2.86% interference rate.

Selecting an assay for the laboratory requires understanding the strengths and limitations of the method. For our work flow, our lab chose the assay with more interferences, since our work flow reflexes presumptive positive immunoassay results and interferences to confirmation by mass spectrometry. For labs that use an immunoassay screen-only approach, selecting an immunoassay that generates a higher rate of interference flags could make result interpretation difficult for the client and raise concerns for specimen adulteration.