2.6 Statistical analyses
If antidepressant use was reported, and plasma concentration was
undetectable, the respective concentration was set at half of the LLQ.
For amitriptyline and nortriptyline, where also semi-quantitative
concentrations below LLQ were available, half of the lowest detectable
concentration was taken. Because the number of participants in the
various individual antidepressant groups were too low for the analyses,
the individual antidepressants subclasses needed to be pooled into two
groups: SSRIs and TCAs. Although venlafaxine is a selective
serotonin-norepinephrine reuptake inhibitor (SNRI), we analyzed
venlafaxine combined with the SSRIs. Since the dose of venlafaxine in
our cohort did not exceed 150 milligram (mg), SNRIs mainly inhibit
reuptake of serotonin and thus closely resemble SSRIs19.
Relationship between drug exposure and fall risk were analyzed with
plasma concentrations expressed on both a continuous and categorical
scale. If a participant used two or more antidepressants concomitantly,
only the antidepressant with the highest concentration was included in
the analyses. First, the continuous plasma concentration levels of the
individual antidepressants were analyzed. After that, we standardized
the concentrations of the different antidepressants by creating z-scores
to be able to combine the continuous concentrations. Second, for each
antidepressant the median of the plasma concentration was calculated and
the concentrations were reclassified into a binary category (below
(which was set as reference) or above the median). Third, to explore the
influence of the highest plasma concentrations, the antidepressant
plasma concentrations were divided into four categories. The reference
category contained the concentrations below the LLQ. After that, the
concentrations above LLQ were equally divided into tertiles. For the
TCAs, the remaining number of participants per category was too low.
Thus the reference category contained the lowest tertile and the
concentrations below LLQ and the middle and highest tertile were
combined. The same was applied for the SSRI follow-up concentrations.
Also, delta concentrations (concentration at follow-up minus
concentration at baseline) were calculated, reflecting concentration
changes over time. These were reclassified into a binary category,
negative (decrease or equal concentration over time) or positive
(concentration increase over time)
Baseline characteristics were compared between fallers and non-fallers
in antidepressant users. Differences between groups were tested using
chi-square test, t-test or a Mann-Whitney U test (categorical and
continuous non-normally and normally distributed data, respectively). To
study the association between the antidepressant plasma concentration at
baseline and falls during follow-up, Cox proportional hazard models were
used to calculate hazard ratios (HRs). First, we built a model adjusted
for age and gender (model 1). Second, the following covariates were
selected as potential confounders based on a Directed Acyclic Graph
(DAG) and included in the models if they changed the HR of the
association by ≥10% (model 2): region, BMI, smoking, alcohol, pain,
depressive symptoms, MMSE, number of medication and estimated glomerular
filtration rate (eGFR). If covariates could not be added, due to the
lack of 10 fall events per covariate 20, only the
unadjusted model is presented. For follow-up visit and delta
concentrations, logistic regression models were used to calculate odds
ratios (ORs) for the association between concentration and fall risk
prior to follow-up visit. The same models as described for the Cox
proportional hazard models were applied.
To assess whether antidepressant plasma concentrations were correlated
with dosage, we calculated Pearson correlation coefficients. Since there
were no signs of multicollinearity (correlation coefficient
<0.5) between plasma concentration and dosage, as a possible
marker for example for disease severity, was added to model 2 to test
whether dosage could be a confounder.
P-values <0.05 were considered statistically significant.
Statistical analyses were performed in SPSS (version 28.0, IBM, Armonk,
NY, USA).