Longitudinal Effects of Changes in Cocaine Use on NfL Levels at Follow-up

At follow-up, CU were characterized as either increasers (participants who elevated their cocaine use between T1 and T2) or decreasers (participants who reduced their cocaine use in the same period). The assignment criteria were based on absolute changes in cocaine concentration in toxicological hair analysis between T1 and T2. The effect of changes in cocaine consumption on NfL levels at follow-up was analyzed using a GLM with Gaussian distribution and log link function. We included group (increasers-/decreasers) and sex (female/male) as fixed factors and age, BMI, and NfL levels at baseline as further covariates. As a second step, Spearman’s correlation coefficients were calculated to investigate dose–response effects of changes in substance use on changes in NfL levels. For this analysis, Δ values were calculated to define changes in objective variables of illicit substance use and self-reported alcohol intake (i.e., Δ_COCAINE=[cocaine concentration in hair at T2]–[cocaine concentration in hair at T1]) and NfL (Δ_NfL=[NfL level at T2]–[NfL level at T1]). Extreme outliers were detected using the 3xIQR detection rule on Δ_NfL values and excluded from the longitudinal analysis. We applied the 3xIQR detection rule, as it is a conservative and robust method for small samples with symmetric distributions [37]. Data from participants, who did not take part to the T2 visit, were excluded from longitudinal analysis.