Contributors to Serum NfL Levels in People without Neurologic Disease

Abstract

Objective: To assess the effects of demographics, lifestyle factors, and comorbidities on serum neurofilament light chain (sNfL) levels in people without neurologic disease and establish demographic-specific reference ranges of sNfL.

Methods: The National Health and Nutrition Examination Survey (NHANES) is a representative sample of the US population in which detailed information on demographic, lifestyle, routine laboratory tests, and overall health status are systematically collected. From stored serum samples, we measured sNfL levels using a novel high-throughput immunoassay (Siemens Healthineers). We evaluated the predictive capacity of 52 demographic, lifestyle, comorbidity, anthropometric, or laboratory characteristics in explaining variability in sNfL levels. Predictive performance was assessed using cross-validated R² (R² _cv ) and forward selection was used to obtain a set of best predictors of sNfL levels. Adjusted reference ranges were derived incorporating characteristics using generalized additive models for location, scale, and shape.

Results: We included 1,706 NHANES participants (average age: 43.6 ± 14.8 y; 50.6% male, 35% non-white) without neurological disorders. In univariate models, age explained the most variability in sNfL (R² _cv = 26.8%). Multivariable prediction models for sNfL contained three covariates (in order of their selection): age, creatinine, and glycosylated hemoglobin (HbA1c) (standardized β-age: 0.46, 95% confidence interval [CI]: 0.43, 0.50; creatinine: 0.18, 95% CI: 0.13, 0.22; HbA1c: 0.09, 95% CI: 0.06, 0.11). Adjusted centile curves were derived incorporating identified predictors. We provide an interactive R Shiny application to translate our findings and allow other investigators to use the derived centile curves.

Interpretation: Results will help to guide interpretation of sNfL levels as they relate to neurologic conditions. ANN NEUROL 2022;92:688-698.

Figure 1.

Consort diagram describing inclusion abd exclusion of study participants.

Figure 2.

Association between age and sNfL levels in 1706 eligible NHANES participants. The fitted blue line denotes a smoothed curve fit estimated using local regression. Scatterplot incorporates survey weights which are depicted by the relative size of the data point; larger datapoints denote individuals with larger weights.

Figure 3.

Univariate associations between candidate predictors and sNfL levels. A. Cross-validated R² between sNfL levels versus a given predictor that are obtained from separated univariate models. Colors denote the classification of predictor type. B. Effect size estimates (95% CI) between a given predictor and sNfL levels. To compare relative associations across predictors, parameter estimates for each continuous predictor are standardized by the ratio of the population standard deviations (SD) of the predictor in question divided by the population SD for sNfL. For binary predictors, the presented effect size denotes the standardized mean difference.

Figure 4.

A. Cross-validated R² at each stage of the forward selection. The dotted vertical denotes time at which stopping criteria was met (e.g., the next additional predictor did not increase the R² by at least 1% [blue; dot-dashed]). Colors denote the classification of the predictor type. B. Standardized predictor estimates for covariates from final multivariable model derived from selection procedure that including age, creatinine, and HbA1c

Figure 5.

Results of GAMLSS model for sNfl that includes age, HbA1c, and creatinine. Values displayed denoted the 95th percentile across ages for sample combinations of identified relevant contributors to sNfL. A. Age, creatinine, and HbA1c-adjusted Z scores across ages demonstrating no association between age and adjusted sNfl Z scores (in contrast to Figure 2) The dotted line denotes Z scores of 1.64. B. Centle curves across ages comparing individuals with kidney disease (e.g., eGFR < 60; mg/dL) versus those without kidney disease (e.g., eGFR≥60; median creatinline = 0.85mg/dL). C. Centile curves across ages comparing diabetics (e.g., HbA1c>6.4%) versus non-diabetics (e.g., HbA1c<5.4%).