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:  103 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 and r = 0.51, respectively). There were differences in subgroups, mostly inconsistent depending on inclusion or exclusion of outliers. Sensitivity and specificity for TLFB detecting presence or absence of cannabis use were 95.7 % (95 % confidence interval 88.0 % to 99.1 %) and 72.2 % (46.5 % to 90.3 %), respectively. Using 19 days as cutoff on TLFB, they were 94.3 % (86.0 % to 98.4 %) and 94.4 % (72.2 % to 99.9 %), respectively. Area under the receiver operating characteristic (ROC)-curve was 0.96. Conclusions:  Time Line Follow-Back (TLFB)-assisted self-report of cannabis use correlates highly with plasma-THC in patients with comorbid cannabis use disorder and psychosis. Sensitivity and specificity of TLFB appear to be optimised with 19 days as cutoff-point. As such TLFB may be superior to analysis of blood when going beyond 19 days of recall.