NEFL and Parkinson disease: Over the first 5 years of the study, NFL increased in both patients with PD and controls: The annual change of log2 transformed NFL for controls was 0.07 (95% CI 0.05–0.08, p < 0.001) and patients with PD had higher levels of log2 transformed NFL at baseline by 0.16 (95% CI 0.04–0.28, p = 0.009) and experienced a faster increase in serum NFL of 0.03 units (95% CI 0.00–0.05, p = 0.029) per year.