There is no blood test for dementia per se. There is a blood test called an ATN panel. It tests for

A - Beta Amyloid 42/40 ratio,
T- Phosphorylated- Tau 181
N- Plasma Neurofilament Light Chain Protein

The combination of reduced Beta Amyloid 42-40 ratio with elevated p-Tau-181 has been associated with presence of Alzheimer's Disease pathology. This panel is not definitive but is suggestive.

You should discuss with your physician.