2.2 Antidepressant usage
Participants were considered antidepressant user based on pharmacy prescription and self-reported usage data. Based on the number of users, participants using TCAs: amitriptyline (N06AA09), nortriptyline (N06AA10), and the SSRIs: citalopram (N06AB04), escitalopram (N06AB10), fluoxetine (N06AB03), fluvoxamine (N06AB08), paroxetine (N06AB05), sertraline (N06AB06) and the serotonin-norepinephrine reuptake inhibitor (SNRI) venlafaxine (N06AX16) 16 were selected.
Usage was defined as having a prescription (based on pharmacy dispensing record data, obtained from the Dutch Foundation for Pharmaceutical Statistics (SFK)). Participants having prescriptions up to 30 days prior blood withdrawal at baseline and/or follow-up visit were selected. To capture more potential users, also participants with a prescription of up to 30 days after the withdrawal date were selected as some participants might have not had a refill in the 30 days before. In case of missing pharmacy data, usage was defined in case of self-reported usage based on questionnaires at baseline and follow-up.
To determine used dosage, defined daily dose (DDD) was used. DDD is a statistical measure of drug consumption and is the assumed average maintenance dose per day for a drug used for its main indication in adults 16. The average prescribed daily dose was based on the prescription (or self-reported data) in closest proximity to the date of baseline blood sampling.