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Fenofibric Acid Can Cause False-Positive Urine Methylenedioxymethamphetamine Immunoassay Results
Loreto Quesada1, Isabel Gomila1, Antonia Fe2
1Servicio de Análisis Clínicos and Unidad de Toxicología Clínica, Hospital Universitario Son Espases, Instituto de Investigación Sanitaria de Palma (IdISPa), Carretera Valldemossa 79, Palma de Mallorca 07120, Spain.
Abstract:
We present a false-positive result of ecstasy (3,4-methylenedioxy-NN-methylamphetamine) screening due to the therapeutic use of fenofibrate, an antihyperlipidemic drug. Our hypothesis was that the main metabolite of fenofibrate, fenofibric acid, was responsible for this cross-reactivity on a DRI(®) Ecstasy Assay, using a cut-off of 500 ng/mL. We estimated that the addition of 225 µg/mL pure fenofibric acid to blank urine would be sufficient to result in a positive DRI(®) Ecstasy Assay. The results obtained on the urine samples analyses of the patient show that the DRI(®) Ecstasy Assay resulted negative 2 days after discontinuing fenofibrate treatment, when the urine fenofibric acid concentration corrected by creatinine and determinated by gas chromatography-mass spectrometry was 20.3 µg/mg creatinine. The cross-reactivity data for fenofibric acid would seem to indicate that there was insufficient concentration of measured compound to account for the positive immunochemical results for ecstasy. This apparent discrepancy can be explained in several ways, one of them is that the β-glucuronidase-resistent fenofibric acid isomers are responsible. This process could explain the low recovery of free fenofibric acid when we use the developed method for its quantification in urine samples. Positive results on immunoassay screening must be considered presumptive until confirmation with another method based on a different principle, preferably gas chromatography-mass spectrometry or liquid chromatography-mass spectrometry.
Insights
Fenofibrate therapy can cause false-positive ecstasy (3,4-methylenedioxy-NN-methylamphetamine) screening results. Fenofibric acid, a metabolite, may be responsible for this immunoassay cross-reactivity.
Area of Science:
- * Clinical Chemistry
- * Forensic Toxicology
- * Pharmacology
Background:
- * Immunoassay screening is widely used for drug detection.
- * Cross-reactivity with therapeutic drugs can lead to false-positive results.
- * Fenofibrate is a commonly prescribed antihyperlipidemic medication.
Purpose of the Study:
- * To investigate the potential for fenofibrate to cause false-positive results in ecstasy screening.
- * To identify the specific metabolite responsible for cross-reactivity.
- * To understand the mechanism behind the observed immunoassay interference.
Main Methods:
- * Analysis of patient urine samples using DRI® Ecstasy Assay.
- * Quantification of urinary fenofibric acid using gas chromatography-mass spectrometry (GC-MS).
- * Correlation of immunoassay results with fenofibric acid concentration post-fenofibrate discontinuation.
Main Results:
- * A patient on fenofibrate therapy exhibited a false-positive ecstasy immunoassay result.
- * Ecstasy assay results became negative after discontinuing fenofibrate.
- * Urinary fenofibric acid levels decreased significantly post-treatment, but cross-reactivity persisted.
- * Potential role of β-glucuronidase-resistant fenofibric acid isomers suggested.
Conclusions:
- * Fenofibrate use can interfere with ecstasy immunoassays, leading to false positives.
- * Fenofibric acid is implicated in the cross-reactivity, possibly via resistant isomers.
- * Confirmatory testing (e.g., GC-MS, LC-MS) is crucial for immunoassay results.
