Clinical and Laboratory Standards Institute document I/LA20-A2--Analytical Performance Characteristics and Clinical Utility of Immunological Assays for Human Immunoglobulin E (IgE) Antibodies and Defined Allergen Specificities; Approved Guideline--Second Edition is written for laboratorians/clinicians (users), manufacturers (producers), and governmental regulators (inspectors, legislators, reviewers). The guideline summarizes the current state of technology, biological specimens that are routinely tested, practical methods for the evaluation of reagents, and clinical interpretation of results. Emphasis is placed on allergen nomenclature and methods for evaluating allergen-containing assay reagents and human IgE-specific, immunologic reagents. Procedures are included for evaluating assay precision, analytical accuracy, sensitivity, parallelism, interference, and documenting the quantitative status of an assay. Guidelines for supplier validation and quality assurance, intralaboratory quality control practices, and interlaboratory proficiency testing are discussed. Finally, performance targets are outlined with recommendations for manufacturers, laboratorians, and users.