NEFL and scrapie: Moreover, the finding that blood samples of asymptomatic scrapie-affected sheep had serum NfL levels similar to those observed in clinically affected sheep suggests that serum NfL increases very early in the disease process and that the Simoa test is a quicker and more efficient tool for the preclinical screening of asymptomatic sheep in flocks at risk for scrapie than 14-3-3 and tau CSF biomarkers [35].