Can a blood test for Alzheimer's disease be trusted after a traumatic brain injury? A blood biomarker is a substance measured in the blood that can provide clues about changes occurring in the brain. Yael Rosen-Lang and colleagues describe in a new paper in JAMA Neurology how a history of traumatic brain injury (TBI) may substantially reduce the accuracy of one of the newest FDA-approved blood tests for detecting Alzheimer's disease pathology. Key points: - The FDA-approved plasma p-tau217/Aβ42 blood test performed very well in older veterans w/o a history of TBI, achieving approximately 90% accuracy for detecting amyloid positivity on PET imaging. - Blood test accuracy declined substantially in veterans w/ prior TBI, particularly in those w/ more severe injuries, where the test missed many individuals who actually had amyloid detected on PET scans. - The findings suggest that a history of TBI should be considered when interpreting Alzheimer's disease blood biomarker results and that additional testing, including amyloid PET imaging, may be needed in selected individuals. My take: Blood biomarkers are transforming how we diagnose Alzheimer's disease. However, this study reminds us that no diagnostic test performs equally well in every clinical setting. A history of traumatic brain injury appears to alter the biology enough that these promising blood tests become less reliable. As we move toward precision medicine, we will need to understand not only what a biomarker measures, but also when and in whom it performs best. This paper is an important reminder that clinical context still matters. Here are 5 points that resonated w/ me: 1- Blood biomarkers are becoming powerful tools for detecting Alzheimer's disease, however they should always be interpreted alongside the clinical history. 2- A prior TBI may substantially reduce the sensitivity of current Alzheimer's disease blood tests, leading to false negative results. 3- Folks w/ previous head injuries should discuss the limitations of blood biomarker testing w/ their health care providers before making important treatment decisions. 4- PET imaging and other confirmatory testing may remain essential for some individuals, especially those w/ a significant TBI history. 5- The future of precision neurology will require tailoring biomarker interpretation to each person's unique medical history, exposures and risk factors rather than relying on a one size fits all approach. cutt.ly/lyw3fTfv