Resolution of an apparent hook effect in Roche partner DRI oxycodone immunoassay

A presumed hook effect in the semiquantitative DRI Oxycodone immunoassay, OXY3S (Cobas Integra, Roche Diagnostics), was investigated in 14 urine samples with gas chromatography/mass spectrometry (GC-MS) >10,000 ng/mL but OXY3S <1,000 ng/mL. These samples included the index case, a false-negative OXY3S result with >75,000 ng/mL oxycodone + oxymorphone by GC-MS confirmation. Patient samples needed 2- to 16-fold dilution to obtain the correct OXY3S response. The OXY3S test did not hook at high-spiked concentrations of oxycodone, oxymorphone or oxymorphone-3β-d-glucuronide in drug-free urine. The OXY3S test parameters were replicated in a development channel on the Cobas using DRI Reagents (Microgenics, CA, USA) and were subsequently modified. Delayed sample addition or doubling of Reagent 1 (R1: antibody/substrate/co-factor) yielded maximal immunoassay response (>10,000 ng/mL) in 12 of 14 and 14 of 14 undiluted patient samples, respectively. Supplementation of R1 with substrate alone did not correctly recover oxycodone from any of the samples, while co-factor supplementation resulted a maximal OXY3S response in 13 of 14 samples. The remaining (index) sample could only be corrected by supplemental R1. The semiquantitative utility of the DRI Oxycodone assay is questionable. Although the precise cause of the under-recovery could not be determined, the modification presented permits reliable oxycodone determination at the high concentrations frequently seen in clinical urine samples.