In the long human effort to see illness before it arrives, a new study of nearly 8,600 people across three countries has found that a blood marker for Alzheimer's disease does not speak the same language to everyone. Published in The Lancet Neurology, the research reveals that a person's genetic inheritance — specifically whether they carry the APOE-ε4 variant — fundamentally changes what elevated p-tau217 levels mean for their cognitive future. The same number on a lab report may signal urgency for one person and a slower unfolding for another, a reminder that biology is always personal befor