CTE can only be definitively diagnosed after death, through examination of brain tissue under a microscope. There is no scan, blood test, or clinical exam that can confirm CTE in a living person. What doctors can do during life is diagnose a related condition called Traumatic Encephalopathy Syndrome (TES), which identifies people whose symptoms and history are consistent with CTE. This distinction matters because it shapes what patients and families can realistically expect from the diagnostic process.
Why Definitive Diagnosis Requires Autopsy
CTE is defined by a very specific pattern of protein buildup in the brain. A damaged form of tau protein accumulates around small blood vessels at the bottom of the brain’s folds (called sulci). These clusters sit in the outer layers of the cortex, which is different from where tau collects in Alzheimer’s disease or normal aging. The molecular structure of these tau fibrils is also unique to CTE, unlike any other brain disease involving tau.
To confirm a CTE diagnosis, a neuropathologist examines brain tissue and looks for four features: perivascular clusters of tau tangles, an irregular distribution of those tangles concentrated at the depths of cortical folds, clusters of tau-affected cells in specific deep brain structures, and tangles located preferentially in the superficial layers of the cortex. No imaging technology available today can detect these microscopic patterns with enough precision to replace direct tissue examination.
The Four Stages Found at Autopsy
When CTE is confirmed after death, pathologists grade its severity on a four-stage scale based on how far the tau protein has spread through the brain.
In Stage I, only one or two isolated spots of tau tangles appear around small blood vessels, usually in the frontal lobe. The brain looks normal to the naked eye. By Stage II, three or more of these lesions appear across multiple brain regions, and tau begins spreading to deeper structures involved in attention and memory.
Stage III marks a significant shift. Tau tangles are now found throughout the memory centers of the brain, including the hippocampus and amygdala. Visible brain shrinkage and enlarged fluid-filled spaces become apparent. By Stage IV, tau has spread extensively through the cortex, brainstem, and even into the spinal cord. Significant loss of brain cells is visible in the frontal and temporal lobes, and pigmented brainstem structures show obvious damage.
How Doctors Evaluate Living Patients
Because CTE itself can’t be confirmed during life, researchers at the National Institute of Neurological Disorders and Stroke developed consensus criteria for diagnosing Traumatic Encephalopathy Syndrome. TES is essentially the clinical picture that suggests CTE may be the underlying cause. Meeting TES criteria doesn’t prove someone has CTE, but it identifies a progressive brain condition linked to repeated head impacts.
A TES diagnosis requires four things. First, the person must have substantial exposure to repetitive head impacts, whether from contact sports, military service, or other causes. Second, they must show core symptoms: problems with memory or executive function (planning, organizing, mental flexibility), behavioral changes like explosive anger, impulsivity, or emotional instability, or both. Third, these symptoms must be getting worse over time. Fourth, the symptoms can’t be fully explained by another neurological, psychiatric, or medical condition.
That last criterion is nuanced. Having another diagnosis like PTSD, depression, anxiety, or even a separate neurodegenerative disease doesn’t automatically disqualify someone from a TES diagnosis. What matters is whether those other conditions fully account for the symptoms. If they don’t, TES can still be diagnosed alongside them. Once diagnosed, doctors grade functional impairment on a scale from independent to severe dementia.
How CTE Looks Different From Alzheimer’s
One of the biggest diagnostic challenges is separating CTE from Alzheimer’s disease, since both involve tau protein tangles and can cause memory loss and cognitive decline. The differences are visible under a microscope but not on a standard brain scan.
In CTE, tau tangles cluster in the outer layers of the cortex (layers II and III) and wrap around blood vessels. In Alzheimer’s, tangles concentrate in the deeper cortical layers (V and VI) and follow a more uniform distribution. The hippocampus is affected in both diseases, but CTE involves tau throughout its subregions (CA1 through CA4), while Alzheimer’s pathology concentrates in CA1. CTE also commonly deposits tau in the substantia nigra, a brain structure involved in movement, which Alzheimer’s rarely affects.
These overlapping yet distinct patterns explain why clinical symptoms alone can’t confirm which disease is present. Some people even have both conditions simultaneously.
PET Scans and Experimental Imaging
Researchers are actively testing brain imaging tools that might one day detect CTE in living patients. The most studied approach uses PET scans with a radioactive tracer called flortaucipir, which was FDA-approved to measure tau tangles in people being evaluated for Alzheimer’s disease. In research settings, scientists are investigating whether this same tracer can pick up the distinct pattern of tau deposits seen in CTE, particularly the clusters at the depths of cortical folds.
The DIAGNOSE CTE Research Project has used flortaucipir PET scans in former professional and college football players to look for these patterns. However, flortaucipir was designed to bind to the type of tau found in Alzheimer’s, and CTE tau has a different molecular structure. This means the tracer may not be sensitive enough to reliably detect early-stage CTE. The technology is promising but remains experimental for this purpose.
Blood Tests Are Not Ready Yet
Blood-based biomarkers represent another frontier. Two forms of tau protein that can be measured in blood plasma, called p-tau181 and p-tau217, have shown strong results for detecting Alzheimer’s pathology. Naturally, researchers wondered whether these same markers could flag CTE.
The results so far are discouraging for that specific use. In a study of patients with Traumatic Encephalopathy Syndrome, p-tau blood levels were good at identifying which patients also had Alzheimer’s-related brain changes, but they did not detect CTE tau on its own. Two study participants who had confirmed high-level CTE pathology without Alzheimer’s changes had some of the lowest p-tau levels in the entire study. The researchers concluded that there is currently no support for using p-tau181 or p-tau217 as biomarkers for CTE tau in living patients.
Quantifying Head Impact Exposure
Since a history of repetitive head impacts is required for a TES diagnosis, researchers have developed tools to estimate how much cumulative force a person’s brain has absorbed. One approach, the Cumulative Head Impact Index, combines a person’s self-reported playing history (years, level of play, positions) with published data from helmet accelerometer studies to estimate total head impact exposure across youth, high school, and college football.
A simpler metric, the Head Impact Exposure Estimate, calculates total hours of contact exposure using a self-report interview, though it excludes playing time before high school. These tools aren’t diagnostic on their own, but higher cumulative exposure scores have been linked to greater rates of depression, apathy, and cognitive impairment later in life. They help clinicians assess how seriously to consider CTE-related diagnoses when evaluating a patient’s symptoms.
What This Means in Practice
If you or someone you know is experiencing progressive memory problems, personality changes, or difficulty with planning and decision-making after years of contact sports or other head impact exposure, the path forward typically involves a thorough neurological evaluation. A specialist will assess cognitive function, review the person’s full exposure history, rule out other conditions that could explain the symptoms, and determine whether TES criteria are met.
The honest reality is that a living person can receive a diagnosis that says their symptoms are consistent with CTE, but confirmation remains impossible without examining the brain after death. Brain banks, such as those run by Boston University and the VA, accept donated brain tissue from former athletes and veterans to advance research. For families seeking answers, postmortem donation is currently the only way to know for certain whether CTE was present.