Evaluation of Detection Techniques and Laboratory Proficiency in Testing for Drugs of Abuse in Urine: An External Quality Assessment Scheme Using Clinically Realistic Urine Samples

Abstract

Techniques for the detection of drugs of abuse were compared using data from 43 urine samples distributed by the United Kingdom National External Quality Assessment Scheme. Drug mixtures simulated clinical specimens and contained drugs at the following median concentrations: amphetamine 1·6 mg/L (six samples), barbiturates 1·7 mg/L (six samples), benzoylecgonine 0·6 mg/L (eight samples), benzodiazepines 1·6 mg/L (four samples), cannabinoids 59 μg/L (four samples), methadone 1·7 mg/L (11 samples) and morphine 1·7 mg/L (seven samples). Significant lack of sensitivity was demonstrated for one or more analytes by all chromatographic techniques, including gas chromatography with mass spectrometry. Commercial immunoassay kits exhibited higher sensitivity, though some assays performed significantly less well than others. Significant non-specificity was also a greater problem for chromatographic techniques compared with immunoassays in samples containing therapeutic agents not structurally related to abused compounds. In the presence of therapeutic interferents structurally related to the test compounds, false positive reports for illicit materials increased from < 1% to > 6%. Use of confirmatory tests significantly reduced the incidence of both false negative and false positive reports.