Correlations and agreement between delta‐9‐tetrahydrocannabinol (THC) in blood plasma and timeline follow‐back (TLFB)‐assisted self‐reported use of cannabis of patients with cannabis use disorder and psychotic illness attending the CapOpus randomized clinical trial

Aims  To assess correlations and agreement between timeline follow-back (TLFB)-assisted self-report and blood samples for cannabis use. Design  Secondary analysis of a randomized trial. Setting  Copenhagen, Denmark. Participants  One hundred and three patients from the CapOpus trial with cannabis use disorder and psychosis, providing 239 self-reports of cannabis use and 88 valid blood samples. Measurements  Delta-9-tetrahydrocannabinol (THC), 11-hydroxy-delta-9-tetrahydrocannabinol (11-OH-THC) and 11-nor-delta-9-tetrahydrocannabinol-9-carboxylic acid (THC-COOH) detected in plasma using high-performance liquid chromatography with tandem mass spectrometry detection. Self-report of cannabis-use last month by TLFB. Pearson's r, sensitivity and specificity calculated as measures of correlation or agreement. Findings  Correlations were strong; r = 0.75 for number of days and r = 0.83 for number of standard joints in the preceding month when excluding outliers. Including outliers, coefficients were moderate to strong (r = 0.49). There were differences in subgroups, mainly inconsistent, depending on inclusion or exclusion of outliers. Sensitivity and specificity for TLFB detecting the presence or absence of cannabis use were 95.7% [95% confidence interval (CI) 88.0–99.1%) and 72.2% (95% CI 46.5–90.3%), respectively. Using 19 days as cut-off on TLFB, they were 94.3% (95% CI 86.0–98.4%) and 94.4% (95% CI 72.2–99.9%), respectively. Area under the receiver operating characteristic (ROC) curve was 0.96. Conclusions  Timeline follow-back (TLFB)-assisted self-report of cannabis use correlates highly with plasma-delta-9-tetrahydrocannabinol in patients with comorbid cannabis use disorder and psychosis. Sensitivity and specificity of timeline follow-back appear to be optimized with 19 days as the cut-off point. As such, timeline follow-back may be superior to analysis of blood when going beyond 19 days of recall.