MAPT and Alzheimer disease: Thus, amyloid positivity (A+), defined as reduced CSF levels of Aβ42, or, preferably, reduced Aβ42/Aβ40 and tau positivity (T+), defined as increased levels of τP181, formulate the CSF profile of AD, typified as A+T+ [5] or A+T1+ (since τP181 increases very early in the AD process, as a result of amyloidogenesis) [6].