Quest AD‑Detect®, Beta‑Amyloid 42/40 Ratio, Plasma

Use

This LC‑MS/MS assay measures beta‑amyloid 40 (AB‑40) as well as AB‑42 in plasma, rather than AB‑42 alone, permitting normalization of circulating beta‑amyloid levels across patients. Using the ratio of beta‑amyloid 42 to beta‑amyloid 40 improves sensitivity and specificity for detecting Alzheimer’s disease pathology. A low 42/40 ratio reflects selective depletion of beta‑amyloid 42 due to amyloid plaque deposition or cerebral amyloid angiopathy. A ratio below 0.160 indicates higher likelihood of amyloid PET positivity, with intermediate ranges correlating with graded probabilities (e.g., 0.169–0.160 corresponds to ~15% PET positivity; 0.159–0.150 corresponds to ~85%).