There is no single test that definitively diagnoses dementia or Alzheimer’s disease. Instead, doctors use a combination of cognitive screening, blood work, brain imaging, and sometimes spinal fluid analysis to build a diagnosis. The process typically unfolds over several visits and can involve your primary care doctor, a neurologist, or a memory specialist. The good news: testing has improved dramatically in recent years, with the first blood test for Alzheimer’s clearing FDA review in early 2025.
How the Diagnostic Process Works
Diagnosing dementia follows a step-by-step approach that narrows down what’s causing cognitive changes. The UCSF Memory and Aging Center outlines it in five stages: identifying a concern, testing cognitive function, ruling out treatable causes, assessing how symptoms affect daily life, and then sharing the diagnosis with a care plan.
The process usually starts when you, a family member, or your doctor notices changes in memory or thinking. For people over 50, experts recommend that doctors ask patients and someone who knows them well every one to two years whether there have been any changes in memory or thinking skills. That initial concern triggers the more formal evaluation.
Some primary care doctors handle the full workup themselves, particularly for straightforward cases. Others refer patients to neurologists or memory specialists. How this plays out depends on the complexity of your symptoms, your doctor’s comfort level, and whether specialists are available in your area. Either way, expect the evaluation to involve multiple appointments over weeks or months rather than a single office visit.
Cognitive Screening Tests
The first concrete step is usually a cognitive screening test, a short set of tasks you complete in the doctor’s office. Several are widely used. The Mini-Cog is a brief two-part test often used as a quick check. The Montreal Cognitive Assessment (MoCA) is more detailed, with a standard threshold score below 26 (out of 30) suggesting mild cognitive impairment, and scores around 18 or below pointing toward Alzheimer’s disease. The Mini-Mental State Examination (MMSE) is another common option, though it uses varying score thresholds depending on the study.
There’s also the Informant Questionnaire on Cognitive Decline in the Elderly, which takes a different approach. Instead of testing you directly, it asks someone who knows you well to rate how your memory and thinking have changed over the past 10 years across a range of everyday tasks, scoring each on a 1-to-5 scale from “much improved” to “much worse.”
The FDA has also cleared several digital cognitive testing tools, including CANTAB Mobile, CognICA, Cognigram, and Cognivue. A device called Cognision uses a headset with scalp electrodes to measure electrical brain activity related to cognitive function. These digital tools are becoming more common but are supplements to, not replacements for, a full clinical evaluation.
No cognitive screening test alone can diagnose Alzheimer’s or any other type of dementia. These tests identify that a problem exists and roughly how severe it is. Figuring out the cause requires additional testing.
Blood Tests and Lab Work
Blood tests serve two purposes in a dementia evaluation. The first is ruling out treatable conditions that can mimic dementia. Your doctor will likely order thyroid function tests, a complete blood count, a metabolic panel, vitamin B12 levels, and calcium levels. Depending on your situation, they may also test for HIV, syphilis, or other infections known to cause cognitive problems. A thyroid disorder or B12 deficiency, for example, can cause memory and thinking problems that improve with treatment.
The second purpose is newer and more exciting: blood tests that detect Alzheimer’s-specific brain changes. In January 2025, the FDA cleared the first blood test for diagnosing Alzheimer’s disease. Called the Lumipulse G pTau217/β-Amyloid 1-42 Plasma Ratio, it measures two proteins in the blood and calculates their ratio. In clinical studies, 91.7% of people who tested positive on this blood test had confirmed amyloid plaques on a PET scan or spinal fluid test. Among those who tested negative, 97.3% were confirmed negative by those same methods. Less than 20% of the 499 patients in the study received an indeterminate result.
This blood test is a significant step forward because, until recently, confirming Alzheimer’s-related brain changes required either a PET scan costing thousands of dollars or a spinal tap. A blood draw is faster, cheaper, and far less invasive. That said, the test is currently intended for people already showing symptoms, not for screening healthy individuals without cognitive complaints.
Brain Imaging
Structural imaging with MRI or CT scans is a standard part of most dementia evaluations. These scans show the physical structure of the brain and can reveal shrinkage in specific regions, evidence of strokes, tumors, or fluid buildup that might explain cognitive symptoms.
More specialized PET scans can detect the hallmark brain changes of Alzheimer’s disease: amyloid plaques and tau tangles. Amyloid PET scans have 88% to 98% sensitivity and 80% to 95% specificity when compared against autopsy findings. Tau PET scans show about 92% sensitivity and 80% specificity for detecting advanced tau pathology. In practical terms, these scans are highly reliable at confirming or ruling out Alzheimer’s-related brain changes.
Medicare covers FDG-PET scans in limited circumstances, primarily when a doctor needs to distinguish between Alzheimer’s disease and frontotemporal dementia in a patient who has had at least six months of documented cognitive decline and meets diagnostic criteria for both conditions. The patient must have already undergone a comprehensive evaluation, including a medical history, physical and mental status exams, lab tests, and structural imaging like MRI or CT. Outside these specific scenarios, Medicare generally does not cover PET scans for dementia diagnosis.
Spinal Fluid Analysis
A lumbar puncture (spinal tap) can measure proteins in the cerebrospinal fluid that indicate Alzheimer’s. People with Alzheimer’s disease have lower levels of a protein called amyloid-beta 42 and higher levels of tau proteins compared to people without the disease. A large meta-analysis found that total tau levels in Alzheimer’s patients averaged about 2.5 times higher than in healthy controls, while amyloid-beta 42 levels were roughly half the normal amount.
Spinal fluid analysis is considered highly accurate, but it’s more invasive than a blood test or brain scan. It’s typically reserved for cases where other testing hasn’t provided a clear answer, or when a definitive biomarker confirmation is needed to guide treatment decisions.
Genetic Testing
Genetic testing can identify whether you carry the APOE4 gene variant, which is the strongest known genetic risk factor for late-onset Alzheimer’s disease. There are three versions of the APOE gene: APOE2, APOE3, and APOE4. Having one or two copies of APOE4 increases your risk, but it is not a diagnosis. Many people with APOE4 never develop Alzheimer’s, and many people without it do.
No gene has been found to directly cause the common, late-onset form of Alzheimer’s disease. Genetic testing tells you about risk, not whether you have or will get the disease. For this reason, it plays a limited role in the diagnostic process and is more commonly used in research settings or for individuals with a strong family history who want to understand their personal risk profile.
How Alzheimer’s Differs From Other Dementias
Dementia is an umbrella term for cognitive decline severe enough to interfere with daily life. Alzheimer’s disease is the most common cause, but vascular dementia, Lewy body dementia, and frontotemporal dementia are also common. The diagnostic workup isn’t just about confirming cognitive impairment. It’s about identifying which type of dementia you have, because the cause affects treatment options and what to expect going forward.
Current diagnostic criteria define Alzheimer’s disease biologically, based on specific brain changes rather than symptoms alone. The disease is understood as a continuum that begins with brain changes in people who feel fine, progresses through stages of increasing damage, and eventually produces noticeable symptoms. Biomarker tests (blood, spinal fluid, or PET scans) are what distinguish Alzheimer’s from other forms of dementia. However, experts currently recommend against biomarker testing in people without symptoms outside of research studies, since no approved treatments exist for that stage.
What to Expect During the Evaluation
Your doctor will also evaluate factors beyond brain disease that could be contributing to cognitive changes. Medications that affect the central nervous system, depression, anxiety, substance use, sleep disorders including sleep apnea, and other medical conditions can all impair thinking and memory. Identifying these is critical because many of them are treatable.
A key part of the evaluation is assessing how cognitive changes affect your daily life. If you can still independently handle all your usual activities, the diagnosis is more likely mild cognitive impairment. If you’ve lost the ability to do things you could previously manage on your own, such as paying bills, driving safely, or managing medications, that points toward dementia. This functional assessment, combined with cognitive testing and biomarker results, is what shapes the final diagnosis.
Throughout this process, doctors gather input not just from you but from someone who knows you well, whether that’s a spouse, adult child, or close friend. That outside perspective is valuable because people in the early stages of cognitive decline often don’t recognize the full extent of changes in their own thinking and behavior.