CPT code 84394 represents a laboratory chemistry assay for measuring total tau, a biomarker used in the evaluation of neurodegenerative diseases such as Alzheimer’s disease. The test matters nationally because total tau measurement is increasingly incorporated into clinical assessment and research workflows to help characterize neuronal injury and disease progression. As biomarker-driven diagnosis and monitoring expand, coverage, coding clarity, and appropriate site-of-service billing become important for laboratories and clinicians.

Key payers included in this overview are Aetna, Blue Cross Blue Shield, Cigna Health, UnitedHealthcare, and Medicare. Readers will find a concise review of the clinical role of the test, expected service settings, common billing modifiers, and the types of documentation and ICD-10 diagnostic contexts that typically accompany orders. The publication also summarizes benchmarks and policy considerations relevant to laboratory billing, including common reimbursement pathways and payer authorization practices where applicable.

This resource is intended for billing professionals, laboratory managers, and clinicians to provide a clear national-level reference for coding and clinical context of CPT code 84394, helping stakeholders align documentation, billing processes, and clinical workflows for neurodegenerative disease biomarker testing.