Introduction
Recent evidence suggests that reduced peripheral levels of brain-derived neurotrophic factor (BDNF) may be involved in the pathophysiology of bipolar disorder (BD), although its relevance in young populations remains uncertain. This systematic review synthesized studies that evaluated serum BDNF levels in children and adolescents with BD, examining its potential as a risk marker. 

Methods
Following PRISMA 2020 guidelines and a protocol registered in PROSPERO, searches were conducted in the Cochrane, MEDLINE, SciELO, and Scopus databases. Studies including participants aged 0–19 years diagnosed with BD according to DSM criteria were included. Studies with mixed samples (adults, children and adolescents) without separate age-group analyses were excluded. 

Results
After screening and eligibility assessment, seven studies were included. Five of them included a control group, from which a meta-analysis was performed. Moderate methodological heterogeneity was observed and corrected after sensitivity analysis, reinforcing the robustness of the findings, although no statistically significant difference in serum BDNF levels was found between patients with bipolar disorder and controls. 

Conclusion
Current evidence does not support BDNF as a diagnostic biomarker for pediatric BD. Future studies with greater sample power and methodological standardization are needed to clarify its role in the risk and course of early-onset bipolar disorder.