CTE has only been diagnosable after death. An Alzheimer's biomarker may change that.

Chronic traumatic encephalopathy is currently diagnosable only at autopsy. That constraint has blocked both the search for a treatment and any honest accounting of who actually has the disease. Findings reported at the Alzheimer's Association International Conference in London on July 15 suggest a protein fragment already used in Alzheimer's research may be the first test that works in living patients. The first thing it could change is whether a CTE treatment can ever be tested.

CTE is a degenerative brain disease linked to repeated head trauma, the kind absorbed across careers in contact sports, military service, and other head-injury exposures. It shows up as mood swings, memory loss, and, in the worst cases, suicidal behavior. The damage is driven by tau, a structural protein that in healthy brains acts as cellular scaffolding; in CTE and related diseases, tau comes unglued and forms tangles that kill neurons. The only way to confirm a case today is to examine that damage after death.

That post-mortem-only requirement has shaped almost everything the public knows about CTE. The most cited prevalence figure, from the Boston University CTE Center, shows CTE pathology in more than 90 percent of 376 former NFL players, comes from donor brains. A separate 2018 review of donated tissue found CTE-like damage in just 1 of 164 brains in a general-population sample, a wide gap that reflects selection bias in who donates, not the true spread of the disease. Without a living diagnostic, no one can enroll patients in a treatment trial, counsel them on prognosis, or even be sure who has the condition.

The new finding, reported by neuroscientist Chihiro Sato of Washington University in St. Louis and colleagues, points to eMTBR-tau243, a tau fragment measurable in cerebrospinal fluid and post-mortem brain tissue. Sato and co-researcher Kanta Horie, both in the same Washington University lab, originally developed the marker for Alzheimer's disease, where elevated eMTBR-tau243 tracks the same kind of tau tangles that drive CTE. The team's analysis suggests the marker can distinguish people with CTE pathology from those without, using samples taken in life. The signal eMTBR-tau243 picks up is the tau fragment common to both diseases, but the pattern differs in ways the marker can read.

John Arena, a neurosurgeon at the University of Pennsylvania who was not involved in the work, framed the practical stakes. A living diagnostic would, he said, "provide clarity to patients about their well-being and be invaluable to getting them involved in future trials of any CTE treatments." That second point is the bottleneck. No CTE therapy has ever been tested in a randomized trial, in part because no one has yet had a way to confirm who has the disease before a candidate drug can be given.

If a working test were validated, the second-order consequences would extend well beyond individual patients. Researchers could finally estimate CTE prevalence in living, representative cohorts, instead of working from the skewed denominator of donor brains. Drug developers could enroll confirmed cases in trials, then track whether a candidate treatment actually slows the disease. Families of athletes and veterans with head-trauma histories could, for the first time, get a real answer instead of a wait-and-see.

The finding is preliminary. It was presented at a single conference session, has not been peer-reviewed, and the underlying samples were drawn from post-mortem tissue. Sato said the team still needs to confirm the signal in a larger dataset before any clinical use. The typical path from a conference finding to a clinical test runs through a peer-reviewed paper, a validation study in a broader cohort, and a clinical assay a hospital can run. The team is now working on that validation step. For now, the work is a proof of principle that one protein, developed for Alzheimer's, may be the first to also catch a disease that has lived almost entirely in the autopsy suite.