NEFL and Parkinson disease: People with PD with poor outcomes had higher levels of plasma NFL, adjusting for age and sex compared to people with PD with good outcomes (β = 4.378, P = 0.016), but levels of plasma p-tau181 were not significantly different between groups, adjusting for age, sex and batch (β = 0.461, P = 0.106) (Fig. 4A and B).