The Drug's the Same but the Range Has Changed: Revisiting Therapeutic Ranges for Older Medications.

Therapeutic drug monitoring is an essential tool for optimizing pharmacotherapy and avoiding toxicity, yet for many older drugs, therapeutic ranges are derived from historical studies that may not reflect contemporary clinical practice. Changing clinical indications, greater recognition of patient-specific factors, and new pharmacokinetic data may support the need for updated ranges. This review examines methodological, clinical, and analytical challenges in defining and updating therapeutic ranges, including heterogeneous study designs, variable clinical endpoints, drug interactions, and interassay variability. Using vancomycin, digoxin, lithium, and first-generation antiepileptic drugs as examples, we highlight how therapeutic targets have evolved despite incomplete adoption by laboratories.