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Amyloid Beta 42/40 Ratio, Plasma

The ratio of two amyloid peptides in blood plasma. A lower ratio can be a sign of amyloid build-up in the brain, and each lab method has its own cutoff.

AI-verified against cited sources. Awaiting expert reviewAlso called Aβ42/40, Aβ42/Aβ40, AB42/40LOINC 106941-8

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What it is

Amyloid beta (Aβ) peptides are small protein fragments. Aβ42 is the sticky form that can clump into plaques in the brain, and Aβ40 is a more common form that clumps less. This test measures both in plasma and divides Aβ42 by Aβ40. When plaques build up in the brain, the ratio in blood tends to drop a little. The ratio is worked out by the lab and has no units.

Why it matters

Clinicians use amyloid tests when someone has memory or thinking changes, to help judge whether Alzheimer's-type brain changes could be part of the picture. A blood test is easier than a spinal fluid test or an amyloid PET scan. But the plasma ratio drops by only about 8 to 15 percent in people with amyloid, compared with 40 to 60 percent in spinal fluid, so small lab differences matter. p-tau217 tests usually separate people better. Experts treat this ratio as one clue, not an answer on its own.

If it’s low

A lower ratio can be a sign of amyloid build-up in the brain, the process linked to Alzheimer's disease. It is not a diagnosis. Some older adults with no symptoms also have amyloid, and results close to a lab's cutoff can go either way.

Common causes

  • Amyloid build-up in the brain (changes can begin years before symptoms)
  • Older age (amyloid is more common, and average ratios are lower, at older ages)
  • A result close to the lab's cutoff, where small measurement differences can change the category
  • Differences in sample handling or method between labs

What can help

  • Talk with your clinician or a memory specialist about whether another test, such as a p-tau217 blood test, an amyloid PET scan or a spinal fluid test, would help
  • Write down memory or thinking changes that you or your family have noticed, and bring the list to your visit
  • Ask which lab and method produced your result and what cutoff it used

If it’s high

A ratio at or above your lab's cutoff is the usual finding in people who do not have amyloid build-up in the brain. It does not rule out other causes of memory or thinking changes.

Common causes

  • No amyloid build-up in the brain
  • Younger age (average ratios are higher in younger adults)
  • Normal differences between people

What can help

  • If you have symptoms, ask your clinician what else could explain them, since this result does not rule out other causes
  • Use the same lab and method if you repeat the test later

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

For adults: 0.095 and above ratio. Source: Mayo Clinic Laboratories Extended Catalog (testcatalog.org).

What can affect your result

  • Method: mass spectrometry and immunoassays give different ratios and need different cutoffs
  • Age: in one lab's study of cognitively normal adults, average ratios were about 0.111 at ages 40–59, 0.102 at 60–69 and 0.096 at 70–85
  • Sample handling: plasma needs prompt processing, and the rules differ by lab
  • Medical conditions that the Alzheimer's Association guideline says may affect blood biomarkers, such as recent acute brain injury, ALS and severe kidney disease
  • Medicines the same guideline names, such as neprilysin inhibitors
  • Sample handling matters: the C2N instructions in the Mayo Clinic Laboratories catalog call for plasma to be spun down and frozen within about two hours of collection, shipped frozen. Warm or delayed handling can change peptide levels.
  • Method: mass spectrometry and immunoassay ratios are not interchangeable, and immunoassay ratios performed less well in head-to-head studies.
  • Conditions the Alzheimer's Association guideline flags as possible confounders for blood biomarkers: recent acute brain injury, severe chronic kidney disease and ALS.
  • Medicines the same guideline flags: neprilysin inhibitors and drugs that disrupt the blood-brain barrier.

When to talk to a clinician

A blood amyloid result is not an emergency on its own. Talk with a clinician if you or your family notice memory or thinking changes that are new, getting worse or getting in the way of daily life. Call 911 or your local emergency number for sudden confusion, trouble speaking or face drooping.

Questions you could ask

  • Is this test appropriate for me, given whether I have symptoms?
  • Should we repeat it, and if so, at the same lab?
  • If I have memory or thinking changes, what else could explain them?
  • Does this result change which tests you would suggest next?
  • Would a p-tau217 blood test, a PET scan or a spinal fluid test help clarify this result?
  • Which lab and method was used, and what cutoff does it apply?

Questions people ask

Why do labs use different cutoffs for this ratio?

Each method measures the two peptides a little differently, so the ratio comes out on a different scale. For example, one lab prints 0.095 as its cutoff, while a research method used 0.1218. Compare results only when they come from the same lab and method.

Can this test tell me whether I will get dementia?

No. It looks at a sign of amyloid in the brain, not at memory itself. Some older adults with no symptoms have amyloid, and the lab that offers this test says it is not meant for people without symptoms. A clinician can help you understand what a result means for you.

Is a blood test as reliable as a PET scan or spinal fluid test?

Not yet, for this ratio alone. The difference between people with and without amyloid is small in blood, so results near the cutoff can be uncertain. p-tau217 tests usually separate the groups better, and specialists often confirm unclear results with a PET scan or spinal fluid test.

Show the science and sources

For clinicians and researchers

Plasma Aβ1-42/Aβ1-40 is reported as a mass ratio (LOINC 106941-8 is defined for LC-MS/MS). Because plasma Aβ also comes from outside the brain, brain amyloid lowers the plasma ratio by only about 8–15% versus 40–60% in CSF, so the test is sensitive to analytical imprecision and pre-analytical variation (Hansson 2022). In a head-to-head study, the Washington University IP-MS ratio reached an AUC of 0.86 for matching CSF Aβ42/40 status, versus 0.69–0.78 for an antibody-free LC-MS method and three immunoassays (Janelidze 2021). Cutoffs are method-specific: C2N prints ≥0.095 as consistent with absence of plaques, while a research mass-spectrometry assay used 0.1218 (Schindler 2019). In C2N's FDA submission (K253240), 464 cognitively normal adults aged 40–85 had a median ratio of 0.1005 (2.5th–97.5th percentile 0.0814–0.13); the ratio feeds the cleared Amyloid Probability Score together with %p-tau217, and the cleared result is that combined score, not the ratio alone. The 2025 Alzheimer's Association guideline covers adults with objective cognitive impairment in specialized care, evaluated 31 blood tests, including Aβ42/40 ratios, using single cutoffs only, and notes that ratios with small fold changes are more susceptible to assay imprecision.

LOINC 106941-8 (primary). Specimen: plasma.

Sources

  1. PrecivityAD2, Plasma (Mayo test ID C2AD2, performed by C2N Diagnostics), Mayo Clinic Laboratories Extended Catalog (testcatalog.org) (2026)
  2. PrecivityAD, Plasma (Mayo test ID C2NAD, performed by C2N Diagnostics), Mayo Clinic Laboratories Extended Catalog (testcatalog.org) (2026)
  3. PrecivityAD2 Test, 510(k) K253240 decision summary, U.S. Food and Drug Administration (2026)
  4. Head-to-Head Comparison of 8 Plasma Amyloid-β 42/40 Assays in Alzheimer Disease, JAMA Neurology (2021)
  5. High-precision plasma β-amyloid 42/40 predicts current and future brain amyloidosis, Neurology (2019)
  6. The Alzheimer's Association appropriate use recommendations for blood biomarkers in Alzheimer's disease, Alzheimer's & Dementia (2022)
  7. Alzheimer's Association Clinical Practice Guideline on the use of blood-based biomarkers in the diagnostic workup of suspected Alzheimer's disease within specialized care settings, Alzheimer's & Dementia (2025)
  8. LOINC 106941-8: Amyloid beta 42 peptide/Amyloid beta 40 peptide [Mass Ratio] in Plasma by LC/MS/MS, LOINC via NLM Clinical Tables

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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