Detection of residues in urine and tissues of sheep treated with trace levels of dietary ractopamine HCl1,2

Qualitative assays are sometimes used as the sole basis for detecting drug residues in live animals or in animal products. Such assays have become increasingly sensitive as detection technologies have improved, yet the limitations of such assays to discriminate purposeful and accidental drug exposures remain poorly defined. A study was conducted to determine the ability of a ractopamine lateral flow assay to accurately detect incurred ractopamine residues in contaminated feeds and in sheep fed trace quantities of ractopamine HCl. False positive and negative samples were determined using a quantitative liquid chromatography-tandem mass spectrometric (LC-MS/MS) method. Ractopamine HCl was fed to sheep at 0 (Zero), 1 (Low), 10 (Med), or 100 (High) µg/kg of diet ( = 4 per level, 0.5 kg of feed/d) for 7 consecutive d and urine was collected daily about ∼16 h post exposure. On-site lateral flow assays were able to reliably (0% false negatives) detect 20 μg of ractopamine HCl per kg of feed. Urine from treated sheep tested positive for ractopamine residues by lateral flow assay in 7.4 (Zero), 0 (Low), 82 (Med), and 86% (High) of the urine samples from each group. Parent ractopamine was below the assay limit of quantification (LOQ, 0.7 ng/mL) in all urine samples using LC-MS/MS. After hydrolysis of ractopamine conjugates, total ractopamine (parent + hydrolyzed metabolites) in urine of Low animals was always less than the LOQ, but in 7 of 28 samples were above the limit of detection (LOD, 0.22 ng/mL). In contrast, urine in Med animals contained 1.08 to 9.13 ng/mL of total ractopamine, while urine of High animals contained 4.85-32.82 ng/mL of total ractopamine. Ractopamine is rapidly eliminated; nevertheless, > 80% of urine samples from sheep exposed to 5 µg/d (M) of ractopamine HCl had detectable residues by the screening assay and a 100% of samples had measurable ractopamine using LC-MS/MS methods. Tissue residues of ractopamine were not detected in any of the sheep. The sensitivity with which the rapid, qualitative assay detected ractopamine was sufficient to reveal trace ractopamine exposures; these data suggest that the use of qualitative tests to indicate purposeful treatment of animals (i.e., for doping or growth enhancement), in the absence of collaborating quantitative data, is inappropriate.