Objective: We investigated diagnostic utility of phosphorylated tau 217 and 181 (ptau217, ptau181), glial fibrillary acidic protein (GFAP), amyloid beta 42 and 40 (Aβ42, Aβ40), and neurofilament light (NfL) to distinguish biomarker-defined Alzheimer disease (AD) from non-AD conditions, in a heterogenous clinical cohort of younger people. Methods: Plasma biomarkers were analysed using ultrasensitive technology, and compared in patients with CSF Alzheimer disease profiles (A+T+) to other CSF profiles (Other). Results: Seventy-nine patients were included, median age 60.8 years: 16 A+T+, 63 Other. Ptau217, ptau181, GFAP were significantly elevated in A+T+ compared to Other (3.67 vs 1.12 pg/mL, 3.87 vs 1.79 pg/mL, 189 vs 80 pg/mL, respectively). ptau217 distinguished AD from Other with 90% accuracy (88% specificity, 100% sensitivity). ptau217 also demonstrated strong diagnostic performance for clinically diagnosed AD. Conclusions: Plasma ptau217 has strong diagnostic performance in distinguishing CSF biomarker-defined AD in a clinically relevant, younger cohort of people with symptoms, adding further weight for a simple diagnostic blood test for AD as a cause of a patient’s symptoms.