Clinical laboratory investigations used to aid antimicrobial chemotherapy of serious infection include routine sensitivity testing and, in the case of those drugs with a narrow therapeutic range, routine assays for therapeutic monitoring to assist with dosage individualization. Tests must be of a sufficiently high quality to be clinically useful. Laboratories ensure quality through standard operating procedures, internal quality control procedures, and participation in external quality assessment (EQA) programs. This article demonstrates how EQA returns to the United Kingdom National External Quality Assessment Scheme for Antibiotic Assays and the activity of the United Kingdom National Quality Assurance Advisory Panel showed marked improvement in the technical quality of assays as exemplified by gentamicin assays. The article also highlights additional quality concerns not subject to EQA.