Blood-Based Biomarkers for Alzheimer Disease: Accuracy, Limits, and Confounders
George Grossberg, MD, reviews how accurate the first FDA-cleared Alzheimer blood test is and which patient factors can skew its results. CONFERENCE REPORTERBlood-based biomarkers are a major advance in...
George Grossberg, MD, reviews how accurate the first FDA-cleared Alzheimer blood test is and which patient factors can skew its results. CONFERENCE REPORTER Blood-based biomarkers are a major advance in diagnosing Alzheimer disease, but like every diagnostic tool, they are not 100% accurate.
That is the message of George Grossberg, MD, the Henry and Amelia Nasrallah Professor in Geriatric Psychiatry at Saint Louis University School of Medicine. At the 2026 Jersey City Psychiatry Conference, Grossberg discussed the first blood-based biomarker test for Alzheimer disease cleared by the US Food and Drug Administration (FDA).
The test combines 2 measures: phosphorylated tau at position 217 (p-tau217) and amyloid-β 42 (Aβ42). 1 How the Test Works Grossberg explained that low Aβ42 levels in blood reflect Aβ42 being deposited as amyloid plaque in the brain. P-tau217 is a very early marker of tau phosphorylation, the process that weakens tau protein in neurons.
Looking at the 2 measures together as a ratio gives a picture of both disease processes. According to Grossberg, a negative result is about 97% accurate in ruling out Alzheimer disease, and a positive result is about 92% accurate. These figures match the FDA's data, which showed 97.3% of patients with negative results had a negative amyloid PET scan or cerebrospinal fluid (CSF) result.
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Among patients with positive results, 91.7% had amyloid pathology confirmed by PET or CSF testing. 1 Confounding Factors Grossberg said certain patient factors can affect the results, particularly for p-tau217, the most sensitive marker in the test. The most important and most frequent confounder is serious kidney failure, which can produce false-positive results.
He noted that many older adults have reduced kidney function as part of aging or other risk factors, without being close to needing dialysis. False positives are a concern mainly for patients with more profound renal impairment. Morbid obesity, which is not rare, is another confounder.
Research has shown that chronic kidney disease and other comorbidities can affect plasma Alzheimer biomarker levels. 2 "Nothing is 100%," said Grossberg. "Even the amyloid PET scan, as well as spinal fluid testing, are not 100% accurate.
Sometimes things are a little bit vague, but nonetheless, this has been a huge development in the Alzheimer's arena." Interpreting blood biomarker results alongside a patient's kidney function, body weight, and clinical presentation can help clinicians avoid misdiagnosis. Dr Grossberg is the Henry and Amelia Nasrallah Professor in Geriatric Psychiatry at Saint Louis University School of Medicine.
References US Food and Drug Administration. FDA clears first blood test used in diagnosing Alzheimer's disease. News release.
May 16, 2025. Accessed October 7, 2026. https://www.fda.gov/news-events/press-announcements/fda-clears-first-blood-test-used-diagnosing-alzheimers-disease Mielke MM, Dage JL, Frank RD, et al. Performance of plasma phosphorylated tau 181 and 217 in the community.
Nat Med . 2022;28(7):1398-1405. Related to this article
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