Co-pathology recognition is reshaping how Alzheimer's diagnosis is being read

Five years ago, the operational question at the end of a memory-clinic workup for an older patient was largely "Alzheimer's, yes or no?" The arrival of validated plasma biomarkers and accessible amyloid PET initially reinforced that frame - a binary biomarker readout for a binary diagnostic question.

What has shifted, gradually but visibly, is the recognition that amyloid-positive does not mean amyloid-only. Three co-pathologies are now routinely considered alongside Alzheimer's biology:

• LATE - limbic-predominant age-related TDP-43 encephalopathy - produces an amnestic syndrome that closely mimics Alzheimer's, frequently coexists with it, and likely contributes meaningfully to the rate of cognitive decline in older patients.
• Vascular contribution - small-vessel disease detectable on MRI is highly prevalent in the same age group as symptomatic Alzheimer's and can independently drive cognitive change.
• Lewy-body co-pathology - α-synuclein pathology is present in a substantial minority of clinically diagnosed Alzheimer's at autopsy, and DLB-like clinical features are increasingly recognised pre-mortem.

The implications are practical. For diagnosis, the question is becoming "what is the mix" - which has implications for prognosis, for symptomatic management, and for how a patient's response to anti-amyloid therapy should be interpreted. For therapeutic development, it sharpens the case for a portfolio of mechanisms - addressed in our Insight on the post-amyloid pipeline as a portfolio bet.

The diagnostic infrastructure to characterise this mix in routine practice - TDP-43 biomarkers, broader Lewy-body assays, MRI quantification of vascular burden - is uneven. The conceptual shift is well ahead of the clinical toolkit. We treat the broader version of this gap - between the screening question the field has largely answered and the staging question that is now operationally binding - in our Insight on why diagnosis is becoming staging.