MAPT and Schnyder corneal dystrophy: However, even if different cut-off values of CSF Aβ42 and ratios with tau and Aβ40 were used to classify HCs with normal CSF values (i.e. Aβ42 above cut-off) and SCD patients with abnormal CSF values (i.e. Aβ42 below cut-off), tau plasma levels did not differ between these groups.