Comparison of Immunoassay Results near Cutoff Values with LC-MS/MS in Individuals with Substance Use Disorder Under Probation

Background: This cross-sectional study compared immunoassay urine drug screening with LC–MS/MS confirmation in individuals with substance use disorder undergoing probation monitoring, focusing on samples with results close to assay cutoff values. Methods: The study was conducted between January and March 2026 in Şanlıurfa, Türkiye, and included 110 individuals followed at an Addiction Treatment Center (AMATEM). Urine samples with at least one analyte result within ±20% of the cutoff value were analyzed. Immunoassay screening was performed using a Beckman Coulter AU480 analyzer, whereas confirmation was performed by LC–MS/MS using a SCIEX Triple Quad 5500+ system. Agreement was evaluated using Cohen’s kappa and McNemar’s test, while diagnostic performance was assessed using 2 × 2 contingency tables. Results: Agreement varied substantially among analyte groups. Buprenorphine showed the highest agreement (κ = 0.47), followed by amphetamines (κ = 0.33), whereas opioids showed very low agreement (κ = 0.081). Benzodiazepines, cocaine, and cannabinoids showed no meaningful agreement; however, benzodiazepine and cocaine findings were limited by very low numbers of positive cases. Immunoassay screening produced notable false-positive results for amphetamines and opioids and false-negative results for cannabinoids and opioids. Conclusions: Immunoassay performance was analyte-dependent and appeared limited in samples with results close to cutoff values. LC–MS/MS confirmation remains important for reliable toxicological interpretation in probation monitoring settings.