Alzheimer disease is the most common cause of dementia in older individuals. Cerebrospinal fluid biomarkers and amyloid positron emission tomography (PET) can accurately detect Alzheimer disease brain pathology, but the perceived risks, costs, and limited availability have contributed to low rates of biomarker testing in the clinic. With recent approvals of disease-modifying, amyloid-targeting therapies, incorporation of biomarkers into clinical practice has become more important for medical decision-making. Fortunately, blood-based biomarkers of Alzheimer disease pathology have advanced rapidly in recent years and are now increasingly used in research, clinical trials, and clinical practice. Blood-based biomarkers are highly scalable and promise to improve accurate diagnosis of Alzheimer disease, with the potential for much greater reach than cerebrospinal fluid or PET tests. Among the blood-based biomarkers, plasma phosphorylated tau 217 (p-tau217) has demonstrated the highest accuracy in detecting amyloid pathology and also reflects tau pathology to some degree.