Analysis of Forensic Specimens for Cannabinoids. II. Relationship Between Blood Δ9-Tetrahydrocannabinol and Blood and Urine 11-nor-Δ9-Tetrahydrocannabinol-9-carboxylic Acid Concentrations*

Data from five years of gas chromatographic/mass spectrometric (GC/MS) quantitation of blood, or blood and matched (i.e. concurrently collected) urine specimens, for cannabinoids have been used for two distinct evaluations. In the present study, we assessed the relationship of blood Δ9-tetrahydrocannabinol (THC) with blood and urine 11-nor-Δ9-tetrahydrocannabinol-9-carboxylic acid (COOH-THC) concentrations. Cannabinoid-containing (i.e. positive) forensic blood specimens (298, 135 with matched urines) were used to test for correlations and other relationships between these analytes. No correlation was found between blood THC and blood or urine COOH-THC concentrations. Significant correlations were found between the log blood and log urine COOH-THC concentration (r = 0.65). The correlations, however, were neither high nor consistent enough to suggest their use in extrapolations of urine to blood concentrations. Others have suggested that a urine COOH-THC concentration in excess of 100 ng/mL may indicate impairment. We observed this would result in 17% of individuals with blood THC less than 1 ng/mL being classified as impaired. The rate of inaccurate interpretations would increase if higher concentrations of blood THC were considered characteristic of impairment. It has been suggested that a ratio of blood COOH-THC/THC less than 4:1 is an indicator of impairment. In this study, the percentage of bloods with a ratio less than 4:1 increased as THC concentrations increased. However, THC concentrations in excess of 10 ng/mL were required before a majority (70%) of the specimens fell below this ratio, suggesting limitations to its use. Blood metabolic ratios (THC/COOH-THC) displayed a biomodal distribution. Because of a lack of control over the time of collection and dose of cannabis used, these data can only be considered suggestive of substantive interindividual variations in the metabolism of THC. These data confirm previous observations that interindividual variation in the metabolism and excretion of THC precludes accurate estimates of the parent drug when only the metabolite concentration is known.