Preclinical Alzheimer’s disease is the earliest stage of Alzheimer’s, when biological changes like amyloid plaque buildup are already happening in the brain but no memory problems or cognitive symptoms have appeared yet. This silent phase can last 20 years or more before a person notices anything wrong. Under the most recent diagnostic guidelines, published in 2024, Alzheimer’s is now defined by its biology rather than its symptoms, meaning a person can technically have Alzheimer’s disease long before they feel any different.
How Alzheimer’s Starts Before Symptoms
Alzheimer’s disease doesn’t begin when someone starts forgetting names or getting lost. It begins years earlier, with two proteins gradually accumulating in the brain: amyloid and tau. Amyloid forms sticky plaques between brain cells, while tau tangles up inside them. Together, these proteins interfere with how brain cells communicate and eventually cause them to die. In the preclinical phase, this process is underway but the brain is still compensating well enough that thinking and memory remain intact.
Research tracking people over time has found that amyloid changes can be detected 20 or more years before the expected onset of cognitive symptoms. Subtle declines in cognitive performance, measurable only with sensitive testing, also begin roughly 20 years before a diagnosis of mild cognitive impairment. For most of that window, though, the person feels and functions normally.
Not everyone with amyloid buildup will go on to develop dementia. Studies using brain imaging in cognitively normal older adults find that somewhere between 10 and 30 percent test positive for amyloid. That proportion climbs with age: from roughly 18 percent of people in their 60s to as high as 60 percent of those over 80. Some of these individuals will progress to symptomatic Alzheimer’s, but others will not, at least within their lifetime.
The Stages of Preclinical Alzheimer’s
In 2011, the National Institute on Aging and Alzheimer’s Association (NIA-AA) proposed a framework that breaks preclinical Alzheimer’s into three stages based on what’s happening biologically:
- Stage 1: Amyloid plaques are present in the brain, but there are no signs of nerve cell damage yet. This is the earliest detectable point.
- Stage 2: Both amyloid buildup and evidence of nerve cell injury (such as elevated tau or brain shrinkage) are present, but thinking and memory still test normally.
- Stage 3: Amyloid, nerve cell injury, and very subtle cognitive changes are all present. These changes are so mild they wouldn’t be noticed in daily life but can show up on detailed neuropsychological tests.
The 2024 update to these criteria went further, formally defining Alzheimer’s as a biological disease rather than a clinical one. Under the revised framework, a single positive biomarker for amyloid (from a brain scan, spinal fluid test, or validated blood test) is enough to diagnose Alzheimer’s disease, regardless of whether symptoms exist. This was a major conceptual shift: Alzheimer’s is now treated as a continuum that starts silently and progresses biologically, with symptoms emerging later but not required for diagnosis.
How Preclinical Alzheimer’s Is Detected
Because there are no symptoms to spot, preclinical Alzheimer’s can only be identified through biomarker testing. The current approach categorizes people using what’s called the ATN system: A for amyloid, T for tau, and N for neurodegeneration (brain cell loss). Each category is rated as positive or negative, creating a biological profile. Anyone who is amyloid-positive falls on the “Alzheimer’s continuum,” even without symptoms.
The gold standard tests are amyloid PET scans and spinal fluid analysis. Amyloid PET uses a radioactive tracer that binds to plaques in the brain, producing images that can be scored on a standardized scale. Values below about 10 on this scale reliably rule out significant amyloid, while values above 30 reliably confirm it. The range in between represents an intermediate zone of early accumulation. Spinal fluid tests measure concentrations of amyloid and tau proteins directly.
Blood tests are a newer and more practical option. A test measuring a specific form of tau protein in the blood (p-tau217) has shown promise for identifying people with amyloid buildup. In a large study published in JAMA Neurology, a blood-based p-tau217 test achieved about 81 percent accuracy compared to PET and spinal fluid as reference standards, with very high specificity (meaning few false positives). The tradeoff is sensitivity: at the most reliable cutoff, the test identified only about 46 percent of all amyloid-positive cases. In other words, a positive result is highly trustworthy, but a negative result doesn’t rule out early disease. This makes blood tests useful as a screening step, though not yet a standalone replacement for PET or spinal fluid analysis.
Who Is Most at Risk
The biggest known genetic risk factor for preclinical Alzheimer’s is carrying one or two copies of the APOE ε4 gene variant. Everyone inherits two copies of the APOE gene, and the ε4 version is associated with greater amyloid accumulation and faster progression. Carrying one copy raises the risk of progressing from preclinical Alzheimer’s to mild cognitive impairment or dementia by roughly 50 to 100 percent, depending on age. The effect peaks between ages 70 and 75, when carriers face about 1.8 times the risk of non-carriers.
For progression specifically to Alzheimer’s dementia (rather than mild cognitive impairment), the genetic effect is even stronger in people over 60. In that age range, carriers of the ε4 variant had roughly 2 to 11 times the risk compared to non-carriers, with the hazard increasing alongside the number of ε4 copies. Interestingly, in people 60 and younger, carrying the variant raised the risk of mild cognitive impairment but did not significantly predict progression all the way to dementia, suggesting that age and genetics interact in complex ways.
Beyond genetics, other factors associated with amyloid buildup in cognitively normal people include older age, cardiovascular risk factors, lower education levels, and a family history of Alzheimer’s. None of these guarantee disease, but they shift the odds.
Subjective Cognitive Decline: The Gray Zone
Some people in the preclinical phase notice something that standard tests don’t yet pick up. This is called subjective cognitive decline: a self-perceived sense that your memory or thinking isn’t as sharp as it used to be, even though you perform normally on cognitive tests. It’s a frustrating in-between state, and it doesn’t always mean Alzheimer’s is the cause. Stress, poor sleep, depression, and normal aging can all produce similar feelings.
However, when subjective cognitive decline occurs alongside positive amyloid biomarkers, it takes on more significance. Neuroimaging studies have found that people with these subjective complaints are more likely to show amyloid and tau deposits, gray matter shrinkage, and disruptions in the brain’s white matter connections. Subjective cognitive decline is thought to emerge relatively late in the preclinical phase, closer to the transition toward measurable impairment, making it a potential early warning sign worth paying attention to.
Treatment Trials Targeting the Preclinical Phase
The long preclinical window has made it a major target for prevention trials. The logic is straightforward: if you can clear amyloid or stop its buildup before it causes irreversible damage, you might prevent or delay symptoms entirely. This is a fundamentally different strategy from treating people who already have dementia.
The largest trial to date in this space was the A4 Study (Anti-Amyloid Treatment in Asymptomatic Alzheimer’s Disease), which enrolled cognitively normal older adults with elevated amyloid on PET scans. Participants received either solanezumab, an antibody designed to bind and clear soluble amyloid, or a placebo every four weeks for 4.5 years. The results, published in the New England Journal of Medicine, were disappointing: solanezumab did not slow cognitive decline compared to placebo. The drug targeted only the soluble form of amyloid rather than the plaques themselves, which may explain the failure.
Other trials are now testing different approaches in preclinical populations, including antibodies that target amyloid plaques directly (the same class of drugs that has shown modest effects in people with early symptomatic Alzheimer’s). Whether intervening before symptoms appear will prove more effective than intervening after remains one of the central open questions in Alzheimer’s research.