C-Reactive Protein in Saliva as a Non-Invasive Marker of Metabolic Syndrome: A Systematic Review and Meta-Analysis

Metabolic syndrome (MetS) is a cluster of conditions that rely on low-grade systemic inflammation and increase the risk of cardiovascular disease, type 2 diabetes, and stroke. C-reactive protein (CRP) has attracted growing interest in saliva as a non-invasive alternative to serum CRP testing, though existing evidence remains inconsistent. This systematic review and meta-analysis evaluated the association between salivary CRP levels and MetS and examined the consistency of findings across populations and methodological approaches. PubMed, the Cochrane Library, and Web of Science were searched up to December 2024 following PRISMA 2020 guidelines. Nineteen studies involving 3265 participants with and without MetS were included. Random-effects meta-analysis demonstrated higher salivary CRP levels in individuals with MetS compared with controls (SMD = 1.02; 95% CI -0.23 to 1.81), with substantial heterogeneity (I² = 98.91%), reflecting variation in population characteristics and saliva collection protocols. Funnel plot assessment did not indicate publication bias. Despite considerable heterogeneity, pooled estimates suggest that individuals with MetS have higher levels of salivary CRP. Longitudinal studies employing standardized methodologies are required before its clinical implementation can be considered.