Tracing neuroinflammation in neurodegeneration: insights from a scoping review on biofluid biomarkers

Neuroinflammation is increasingly recognized as a key pathological process in neurodegenerative disease and can be monitored using biofluid biomarkers. Objective biomarkers may aid diagnosis, prognosis and progression. We conducted a scoping review of neuroinflammation biomarkers across major neurodegenerative diseases covering the past 23 years, including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, frontotemporal dementia, Huntington's disease, Lewy body dementia, multiple system atrophy and progressive supranuclear palsy. PubMed and Web of Science were systematically searched for observational studies from 2003 to 2025 reporting neuroinflammation biomarkers in adult human subjects. Included markers encompassed blood, cerebrospinal fluid, saliva and urine, providing possible complementary information. Original studies on non-neuroinflammatory mechanisms, cellular or post-mortem biomarkers, animal models, genetics and comparisons between diseases were excluded. Two reviewers independently screened articles; biomarkers reported in ≥3 independent cohorts per disease were analysed. A total of 388 studies were included, predominantly in Alzheimer's disease/mild cognitive impairment (n = 214) and Parkinson's disease (n = 92). Eight biomarkers were most frequently reported: IL-6, TNF-α, IL-1β, CRP/hs-CRP, IL-10, MCP-1, YKL-40 and neutrophil-to-lymphocyte ratio (NLR), measured in blood or cerebrospinal fluid (CSF) as indicators of inflammatory processes associated with neurodegeneration. Across biomarkers, the strength and scope of evidence varied. Most studies demonstrated higher biomarker levels in disease, with more advanced stages, greater clinical severity and faster progression. NLR showed the most consistent pattern across staging, severity and progression, but is currently under-represented across diseases. CSF YKL-40 generally increased with disease presence and advancement; IL-6 showed consistent increases in advanced stages and with severity, although significant results were limited; MCP-1, CRP and TNF-α were mostly linked to severity and progression; IL-1β and IL-10 remained largely inconsistent. Other markers, including GFAP, showed associations in Alzheimer's disease but remain underexplored in other neurodegenerative diseases. Variability across studies, including differences in biofluid source, assay sensitivity, population characteristics and statistical approaches, limits interpretability and comparability. Although neuroinflammation is elevated in neurodegenerative diseases and generally intensifies as these diseases progress, potentially contributing to downstream pathology, the precise timing, role and predictive value of these biomarkers remain uncertain. A subset of markers, including NLR, YKL-40 and GFAP, shows relatively consistent associations and may warrant further investigation across diseases. In clinical practice, neuroinflammation biomarkers could serve as complementary tools to capture inflammatory processes related to disease heterogeneity and progression. Future longitudinal studies tracking pre-symptomatic and early-stage individuals, with standardized approaches, are needed to define temporal dynamics and explore their utility for monitoring disease progression and therapeutic response.