Challenges in the interpretation of amphetamine results: differences of reactivity between immunoassay and...

Urine drug screening is a valuable diagnostic tool for assessing acute intoxications in the emergency department. Enzyme immunoassay (IA) and immunochromatography (IC) are the two main techniques routinely used due to their rapid turnaround, low cost, and high sensitivity. However, results must be interpreted with caution, as structural cross-reactivity may cause false-positive (FP) findings. We retrospectively analyzed 55 urine samples from emergency patients initially screened for amphetamines by IA and confirmed by gas chromatography-mass spectrometry (GC-MS). Samples containing amphetamine derivatives or potentially cross-reactive substances (tetracaine, trazodone, methylphenidate, ritalinic acid, phentermine, procainamide, ephedrine, bisoprolol, and synthetic cathinones) were subsequently analyzed using an IC assay. Diagnostic performance and cross-reactivity were compared between both screening methods. Of the 55 samples selected by IA, 34 were true positives (TP), 8 true negatives (TN), 2 false negatives (FN), and 11 FP. FP by IA were mainly due to tetracaine, trazodone, phentermine, ephedrine, and procainamide. In comparison, IC yielded 32 TP, 17 TN, 2 FP (phentermine and bisoprolol), and 4 FN. No cross-reactivity with methylphenidate and synthetic cathinones was observed in either technique. IC showed frequent misclassifications between amphetamine and methamphetamine while MDMA detection was more specific. IC is a reliable and rapid alternative for amphetamine screening in the emergency setting, providing improved specificity and fewer false positives than IA. Given its limited discrimination between amphetamine and methamphetamine, IC results should be reported as 'amphetamine derivatives'. These findings highlight assay-dependent cross-reactivity and the importance of understanding methodological limitations for an accurate interpretation.