Is prior traumatic brain injury (TBI) associated with the accuracy of Alzheimer's disease biomarkers? A cross-sectional study of 272 older Veterans (31% amyloid-PET positive) found that while the plasma pTau217/Aβ42 ratio test is highly accurate (90%) for those without a history of TBI, its accuracy drops significantly to between 63% and 78% in those with a prior TBI. This drop is primarily driven by lower sensitivity, meaning the blood test could miss more than 50% of amyloid-PET positive cases in individuals who have experienced a TBI. TBI history and severity act as modifiers of AD blood test accuracy, highlighting the need for cautious clinical interpretation when diagnosing Alzheimer's in patients with past head injuries.