Amyloid beta peptide (Aβ) 40 and Aβ42 levels in cerebrospinal fluid (CSF)

Why This Test Is Done

Doctors may order this test if you have symptoms of Alzheimer’s disease, such as memory loss, confusion, or behavioral changes. It may also be used to evaluate other conditions affecting the brain, like vascular dementia or Lewy body dementia. Sometimes, it’s part of a broader evaluation for suspected neurodegenerative diseases.

What to Expect During the Test

A lumbar puncture (spinal tap) is needed to collect cerebrospinal fluid (CSF) from your lower back. You’ll lie on your side, and a doctor will insert a needle into your spine to safely draw the fluid. You may feel a brief pinch, but the procedure is usually quick. You might experience mild soreness afterward.

What the Results Mean

The test measures two key proteins: Aβ42 (a marker often lower in Alzheimer’s) and Aβ40 (a reference protein). While abnormal ratios can suggest amyloid plaques (common in Alzheimer’s), results alone aren’t diagnostic. They’re usually interpreted alongside clinical findings and imaging.

Common Causes of Abnormal Results

Elevated or low levels of Aβ42/Aβ40 can occur due to other conditions, such as infections, inflammation, or genetic factors unrelated to Alzheimer’s. False results may also happen if the sample isn’t handled properly or if the test isn’t performed correctly.