Plasma p-tau217 / Abeta42 Ratio (Lumipulse)
A calculated ratio of two plasma proteins, p-tau217 divided by amyloid beta 42. A higher ratio makes amyloid build-up in the brain more likely.
AI-verified against cited sources. Awaiting expert reviewAlso called p-tau217/Aβ42, pTau217/AB42, pTau217/Abeta42LOINC 111517-9
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What it is
This number is worked out from two Lumipulse blood tests run on the same plasma sample. p-tau217 tends to go up, and amyloid beta 42 tends to go down, when amyloid builds up in the brain, so dividing one by the other makes the difference larger. The result is a small decimal with no units. The FDA cleared this version in 2025 for adults with signs of cognitive decline who are seen in a specialized care setting.
Why it matters
Clinicians use this ratio as an aid in judging whether amyloid build-up in the brain could explain memory or thinking changes. It sorts results into negative, intermediate or positive. In the FDA study, about 97 of every 100 negative results and about 92 of every 100 positive results matched amyloid PET or spinal fluid testing, and about 1 in 5 results fell in the middle. It does not establish a diagnosis of Alzheimer's disease, and it is not meant for people who have no symptoms.
If it’s low
A lower ratio means amyloid build-up in the brain is less likely. At 0.00370 or below, the FDA-cleared test calls the result negative. Other causes of memory or thinking changes should still be looked into. In the FDA study, about 3 of every 100 negative results still matched positive amyloid testing.
Common causes
- No amyloid build-up in the brain
- Another cause of memory or thinking changes
What can help
- Ask your clinician which other causes of your symptoms should be looked into
- Tell your clinician if your symptoms change or get worse
If it’s high
A higher ratio means amyloid build-up in the brain is more likely. At 0.00738 or above, the FDA-cleared test calls the result positive. This does not establish a diagnosis of Alzheimer's disease. Some older adults with no symptoms also have positive results: about 18 of every 100 cognitively normal adults aged 51 to 84 in the FDA study.
Common causes
- Amyloid build-up in the brain linked to Alzheimer's disease
- Early amyloid changes in an older adult who has no symptoms yet (more common with age)
- Other conditions that raise p-tau217, such as stroke, traumatic brain injury, Lewy body dementia or frontotemporal dementia
- Reduced kidney function, which raises p-tau217 in other studies
What can help
- Ask your clinician how to combine this result with your symptoms, exam and other tests
- Ask whether an amyloid PET scan or a spinal fluid test would help confirm it
- Tell your clinician about other conditions such as kidney disease, stroke or a past brain injury
What can affect your result
- Method: the cutoffs apply only to the Lumipulse pTau 217 and β-amyloid 1-42 assays run on the same sample
- Age: positive ratios were more common at older ages in cognitively normal adults (about 2% of those aged 51–65, 18% overall, and 46% of the 28 adults aged 76–80)
- Other brain conditions: stroke, brain injury, Lewy body dementia and frontotemporal dementia often gave positive ratios
- Sample handling: plasma degrades quickly at room temperature
- How common amyloid is in the group tested, which changes how often a positive result is right
- In the FDA studies, bilirubin, hemoglobin, triglycerides, biotin (tested up to 3,500 ng/mL), rheumatoid factor and many common drugs did not interfere (less than 10% change).
- The cleared test uses K2-EDTA plasma. The FDA summary says samples degrade quickly at room temperature, so storage and shipping at room temperature should be avoided.
- Other conditions that raise p-tau217, such as stroke, traumatic brain injury, Lewy body dementia and frontotemporal dementia, often gave positive ratios in the FDA's study of those groups.
- Chronic kidney disease raises plasma p-tau217 in other studies. The FDA summary does not give a separate kidney analysis for this ratio.
- The cutoffs apply only to Lumipulse G pTau 217 Plasma and Lumipulse G β-Amyloid 1-42-N Plasma run on the same sample. Ratios from other methods are not comparable.
When to talk to a clinician
A blood p-tau217/Aβ42 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 my age and symptoms?
- Which lab and method was used, and does it match any earlier results?
- Should we confirm this result with an amyloid PET scan or a spinal fluid test?
- Could another condition, such as kidney disease or a past stroke, affect this result?
- What other causes of my symptoms should we look into?
- If my symptoms change, should we repeat the test or try a different one?
Questions people ask
Why is this number so small?
It divides a small number (p-tau217) by a larger one (amyloid beta 42), so the result is a decimal such as 0.005. At the FDA-cleared cutoffs, 0.00370 or lower is negative and 0.00738 or higher is positive.
What if my result is in the middle?
A result from 0.00371 to 0.00737 is called intermediate, which means the test cannot say either way. In the FDA study, about half of the people in this zone had amyloid on PET or spinal fluid testing. The labeling says these people should be considered for further testing.
Is this the same as a p-tau217 test on its own?
No. This ratio adds amyloid beta 42 to p-tau217 and has its own cutoffs. A p-tau217 result from another lab uses a different scale and different cutoffs, so the numbers cannot be swapped.
Show the science and sources
For clinicians and researchers
The Lumipulse G pTau 217/β-Amyloid 1-42 Plasma Ratio (Fujirebio; FDA 510(k) K242706, cleared 2025-05-16) is pTau 217 (pg/mL) divided by β-amyloid 1-42-N (pg/mL), each measured by chemiluminescent enzyme immunoassay on the LUMIPULSE G1200 using K2-EDTA plasma. The operator calculates the ratio, because the instrument does not report it. Result of 0.00370 or less is negative, 0.00371 to 0.00737 intermediate and 0.00738 or more positive; the labeling says to round the ratio to 5 decimal places before comparing. Measuring intervals are 0.047–10.000 pg/mL for pTau 217 and 0.8–500.0 pg/mL for β-amyloid 1-42, with rules for values outside them. The clearance document states an indication for adults aged 50 and older with signs and symptoms of cognitive decline at a specialized care setting, while FDA's announcement said 55 and older; the pivotal study enrolled ages 55–93. In 499 symptomatic adults compared with amyloid PET or CSF (prevalence 51.1%), positive results (43.9% of results) were 91.8% concordant, intermediate results (19.6%) 50.0% and negative results (36.5%) 2.7%. In 240 cognitively normal adults aged 51–84, 50.4% were negative, 31.7% intermediate and 17.9% positive, with positives increasing with age (2.5th–97.5th percentile of the ratio 0.00179–0.02609). Groups with stroke, brain injury, Lewy body dementia or frontotemporal dementia often had positive ratios. The test is not intended as a screening or stand-alone diagnostic test. The 2025 Alzheimer's Association guideline did not assess ratios across protein types or two-cutoff tests.
LOINC 111517-9 (primary). Specimen: calculated.
Sources
- Lumipulse G pTau217/β-Amyloid 1-42 Plasma Ratio, 510(k) K242706 decision summary, U.S. Food and Drug Administration (2025)
- FDA Clears First Blood Test Used in Diagnosing Alzheimer's Disease, U.S. Food and Drug Administration (2025)
- 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)
- LOINC 111517-9: Tau protein.phosphorylated 217/Amyloid beta 42 peptide [Ratio] in Serum or Plasma, 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.