| Sumario: | Background: Environmental tobacco smoke (ETS) exposure is a preventable risk factor for adverse health outcomes in early childhood. Urinary cotinine is widely used as a non-invasive biomarker of ETS exposure; however, variation in reported cut-off concentrations limits comparability across studies. No prior systematic review has quantitatively synthesised urinary cotinine cut-off values for children under five years. This review aimed to systematically summarise and evaluate these cut-off concentrations in early childhood. Methods: A systematic review and meta-analysis were conducted in accordance with Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA 2020) guidelines and a prospectively published protocol (PROSPERO CRD42024556969). PubMed, Embase, Scopus, and the Cochrane Library were searched from inception to February 2025. Observational and experimental studies reporting urinary cotinine cut-off concentrations to classify ETS exposure among children under five years were included. Study quality was assessed using the Newcastle–Ottawa Scale. Random-effects meta-analyses were performed to estimate pooled urinary cotinine cut-off concentrations for ETS exposure classification, stratified by unit of measurement (ng/mL vs ng/mg creatinine). Results: Twelve studies were included. Reported urinary cotinine cut-off concentrations ranged from 0.05 to 30 ng/mL for unadjusted values and from 2.47 to 120 ng/mg creatinine for creatinine-adjusted values. Meta-analysis yielded pooled reference values of 5.90 ng/mL (95% CI: 5.53–6.27) and 50.36 ng/mg creatinine (95% CI: 47.53–53.20), respectively. Conclusion: Urinary cotinine is a valid biomarker for assessing ETS in young children, but inconsistent cut-off values limit comparability. The pooled estimates may aid interpretation in surveillance and research but should not be used as universal thresholds. Standardized, age-appropriate cut-offs are needed to strengthen exposure assessment and inform child-focused tobacco control policies.
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