Nineteen studies (23 datasets, n=1,504 participants, range 10 to 79) were included in the review, including 10 case-control studies and 13 clinical trials (mostly non controlled). Loss to follow-up was only reported for one study (40%). The test for heterogeneity was significant, so a random-effects model was used for all meta-analysis.
Brain-derived neurotropic factor level was significantly higher after antidepressant treatment than before in depressed patients (SMD 0.62, 95% CI 0.36 to 0.88; I2=61%). A sensitivity analysis showed the result did not significantly change after the exclusion of any one study. Egger’s test was not significant, indicating an absence of publication bias. A meta-regression showed that age, gender, baseline depression, case-control versus clinical trial, ELISA kit used, and type of antidepressant treatment were not associated with outcome (i.e. p>0.05). There was a significant association between brain-derived neurotropic factor level and: depression symptoms change (p=0.02); period of treatment (p=0.01); previous drug use with a two week cut-off (p=0.004); and previous drug use with a four week cut-off (p=0.02).
Brain-derived neurotropic factor level was significantly higher in healthy people than in patients with depression before treatment (SMD 0.91, 95% CI 0.70 to 1.1) and was also higher for healthy people compared with patients with depression after treatment (SMD 0.34, 95% CI 0.02 to 0.66), indicating that brain-derived neurotropic factor level was significantly higher in healthy people. Meta-regression found significant associations between brain-derived neurotropic factor level both baseline depression (p=0.02, pre-treatment versus healthy people) and age (p=0.02, post-treatment versus healthy people ).
Further subgroup analyses were available as online supplementary material. A subgroup analysis compared drug treatment (eight studies) versus non-drug treatment studies (six studies) and found that, for both types of study, brain-derived neurotropic factor level was significantly higher post-treatment in depressed patients but there was significant heterogeneity in the drug treatment studies (SMD 0.64, 95% CI 0.34 to 0.93; I2=64%) compared with the non-drug treatment studies (SMD 0.32, 95% CI 0.02 to 0.61; I2=5%).