NEFL and Parkinson disease: Interleukin-6 (IL-6) levels were significantly higher in PD-CI (SMD = 0.20, 95% CI: 0.01, 0.92).<h4>Conclusion</h4>This NMA identifies Cys C, GDNF, NfL, and IL-6 as promising blood-based biomarkers for detecting cognitive impairment in PD.