MAPT and Alzheimer disease: Elevated plasma P-tau 181 levels in IIIa and IIIC and CSF in IIIc, in the absence of beta amyloid pathology in CSF (IIIC) and at neuropathology (IIa), align with findings of elevated CSF and plasma P-tau 181 and P-tau 217 in sporadic CJD.18 Given the young age of IIIa and IIIc the elevated P-tau is likely to represent a secondary rather than primary tauopathy, supporting the notion that P-tau elevation may be a marker of extra-neuronal amyloid accumulation and should not just be considered as reflective of AD-amyloid associated changes in tau phosphorylation.