If you have mild cognitive impairment and want to know which test to pursue in 2026, the practical answer is the blood test: a pTau217 plasma assay is the one you can actually get this year, while Tauklarify tau PET exists on paper but is not yet bookable. Tauklarify is a radioactive tracer injected before a PET brain scan to show tau neurofibrillary tangles — the clumped protein deposits that track how far Alzheimer's pathology has advanced — and the FDA approved it in August 2026 for adults with cognitive impairment being evaluated for Alzheimer's.
Mild cognitive impairment (MCI) means measurable memory or thinking problems that have not yet taken away your independence. It is also the stage where both of these tests are at their weakest, in opposite directions. Knowing which weakness applies to you is the difference between a useful result and an expensive, inconclusive one.
Medical information disclaimer: This article is for general educational purposes only and does not provide medical advice, diagnosis, or treatment. Always consult a physician or other qualified health professional about symptoms, medications, tests, or treatment decisions.
Table of Contents
- What Tauklarify was approved to do
- Why you probably cannot book the scan
- The blood test that is actually shipping
- What a negative blood result is worth, and what a positive one is not
- A practical sequence for someone with MCI this year
- Why two tests, and what tau adds that amyloid does not
- Frequently Asked Questions
What Tauklarify was approved to do
The FDA approved Tauklarify (florquinitau F 18 injection, previously known as MK-6240) to identify patients with tau neurofibrillary tangle pathology, according to Lantheus's approval announcement. Its safety and effectiveness for non-Alzheimer's tauopathies — other diseases that also involve tau — are not established, so the scan is an Alzheimer's evaluation tool, not a general tau test. The approval rested on two blinded reader studies covering scans from 617 people: 152 with mild Alzheimer's dementia, 157 with MCI, and 308 who were cognitively unimpaired. As reported by Practical Neurology, readers reached positive percent agreement of 68%–88% and negative percent agreement of 93%–99% against the reference standard.
Read that positive figure carefully. At the low end, roughly a third of scans that should have read positive did not, depending on the reader. The labeling says plainly that a positive or negative scan does not independently confirm or exclude tau pathology, and that performance may be lower in patients at earlier stages of the pathological spectrum — which is where most MCI patients sit. Reported adverse reactions were headache, nausea, injection-site reactions, dizziness and abdominal discomfort.
Why you probably cannot book the scan
Approval and availability are separate events. Lantheus said it will continue supplying the agent to Alzheimer's therapeutic programs through its Pharma Solutions business while it evaluates the path to broader commercial availability. In practice that means the tracer currently flows to drug trials and research programs, not to your local imaging center's schedule. Payment is the second barrier.
CMS removed national coverage determination 220.6.20 effective October 13, 2023, which ended coverage-with-evidence-development and the once-per-lifetime cap on beta-amyloid PET and handed the decisions to regional Medicare Administrative Contractors. The practical consequence in the CMS decision memo is that amyloid PET coverage now varies by contractor — and no national determination covers tau PET at all. So if a clinician proposes a tau PET scan, two questions come before the appointment: can this center obtain the tracer, and what will my contractor pay. Neither has a national answer right now.
The blood test that is actually shipping
Ten days after the Tauklarify approval, the FDA cleared Roche's Elecsys pTau217 plasma immunoassay, developed with Eli Lilly. Roche describes it as the first single-biomarker blood test supporting both rule-in and rule-out of amyloid pathology using the same cutoffs in primary and specialty care. Note the target: this test measures amyloid pathology, not tau burden, even though the molecule it measures is a tau fragment. The clearance covers adults aged 55 and older with signs, symptoms or complaints of cognitive decline.
It returns three possible answers — positive, intermediate, or negative — and it is explicitly not a stand-alone test. An intermediate result is indeterminate and sends you on to confirmatory testing such as PET or spinal fluid analysis. Access arrived fast. Quest Diagnostics said on August 24, 2026 that it would launch an AD-Detect service built on the cleared assay in the fourth quarter of 2026, and Labcorp announced it would offer the test across primary and specialty care. That is the inverted picture worth holding onto: the blood test is reaching MCI patients faster than the scan.
What a negative blood result is worth, and what a positive one is not
The validation numbers explain the whole strategy. In 2,148 people across five cohorts measured against amyloid PET, Elecsys pTau217 showed 98.98% sensitivity but 29.17% specificity in cognitively impaired participants, with a negative predictive value of 92.51% at a rule-out cutoff below 0.189 pg/mL.
Translated: The Alzheimer's Association's first clinical practice guideline on blood biomarkers, published in 2025, sets the bar these numbers must clear. The guideline from Palmqvist and colleagues restricts diagnostic use of blood biomarkers to patients who already have cognitive impairment and are being seen in specialty care, requires at least 90% sensitivity and 75% specificity for triage use, and demands at least 90% on both before a blood test may replace amyloid PET or spinal fluid testing. It also insists results be interpreted inside a full clinical evaluation, not read off a lab slip.
- A negative result is strong information. Very few people with amyloid pathology slip past this test, so a negative reading makes Alzheimer's an unlikely cause of your symptoms and redirects the workup elsewhere.
- A positive result is weak information on its own. With specificity near 29% in impaired patients, many positives belong to people without the pathology, which is exactly why the clearance forbids using it alone.
- An intermediate result is not a mild positive. It means the test did not answer, and confirmatory PET or spinal fluid testing is the next step.
A practical sequence for someone with MCI this year
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- Ask for the pTau217 blood test through a specialist, not as a self-ordered screen. The guideline limits diagnostic use to cognitively impaired patients in specialty care, and it is a triage tool inside a workup that includes cognitive testing and a clinical history.
- Decide in advance what a negative result changes. If it would prompt a search for other causes — sleep apnea, medication effects, thyroid problems, depression, vascular disease — the test earns its place.
- Treat a positive as a referral, not a diagnosis. Ask your clinician which confirmatory test follows, and whether their region's Medicare contractor covers amyloid PET, since the national determination no longer exists.
- Do not wait on tau PET for a treatment decision. Tauklarify's commercial path is still being evaluated, so clinical trial participation is currently the realistic route to a tau scan.
- Ask what the result would mean for eligibility. Anti-amyloid therapy decisions turn on confirmed amyloid pathology, so an indeterminate blood result does not move that question forward.
Why two tests, and what tau adds that amyloid does not
The two tests answer different questions. The blood assay asks whether amyloid pathology is present — a yes-or-no gate. Tau PET asks how far neurofibrillary tangles have spread, which tracks more closely with the severity and timing of symptoms.
That is why drug developers want tau imaging and why the tracer currently goes to therapeutic programs. For an individual with MCI, though, the staging question is the one least well served today. The scan's own labeling warns that performance may be lower at earlier stages of the pathological spectrum, and the blinded reader agreement ran as low as 68% positive agreement. A test built to measure progression is least certain at the point where progression has barely begun.
Frequently Asked Questions
Does Tauklarify test for the same thing as the pTau217 blood test?
No. Tauklarify images tau neurofibrillary tangles in the brain. The Elecsys pTau217 blood test was cleared to support rule-in and rule-out of amyloid pathology, a different hallmark, even though it measures a tau-derived protein in blood.
Will Medicare pay for a tau PET scan?
No national determination covers tau PET. CMS removed NCD 220.6.20 effective October 13, 2023, leaving even amyloid PET coverage to regional Medicare Administrative Contractors, so coverage varies by contractor.
Can I order the pTau217 blood test myself?
The 2025 Alzheimer's Association guideline restricts diagnostic use to patients with cognitive impairment seen in specialty care, with results interpreted within a full clinical evaluation. Quest and Labcorp are making the test available through primary and specialty care channels.
What should I do with an intermediate blood test result?
Treat it as no answer. The clearance states that an intermediate result is indeterminate and requires confirmatory testing such as PET or spinal fluid analysis.
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