Reanalysis of forensic urine specimens containing benzoylecgonine and THC-COOH

Drug testing laboratories are often requested to retest specimens that have tested positive. The reproducibility of analytical retest data for delta-9-tetrahydrocannabinol-9-carboxylic acid (THC-COOH) and benzoylecgonine in specimens that had previously been analyzed and then frozen by the Navy Drug Screening Laboratory, Great Lakes is examined in this study. All specimens were tested by gas chromatography/mass spectrometry (GC/MS). Retest values generally showed a decrease in concentration but exhibited considerable variability. Eighty-five THC-COOH positive urine specimens stored frozen for 1 to 10 months (average 2.3 months) declined an average of 25% (range, +30% to -80%) and 61 benzoylecgonine positive urine specimens stored for 1 to 8 months (average 2.3 months) declined an average of 19% (ranged +20% to -100%) from initial GC/MS test results. Drugs were found to partition into strata when frozen in urine because of the thermodynamics of the freezing process. To assure a homogenous solution for repeat testing, specimens that have been frozen and thawed were gently mixed before analysis.