Qualitative results in proficiency testing: a systematic review of evaluation practices and error integration

Proficiency testing and external quality assessment (PT/EQA) are essential for reliable laboratory diagnostics, however evaluation of qualitative results remains poorly standardized. This systematic review examined how qualitative PT/EQA performance is evaluated and reported across life science domains, with emphasis on performance criteria, acceptance rules, and error handling. Searches of MEDLINE, Embase, Scopus, and Web of Science Core Collection identified 178 eligible studies published between 2016 and 2025. Evaluation practices varied considerably. Binary thresholds were used in 86 % of studies and were most often defined by program organizers (52 %) or formal standards (24 %), while 49 % did not report a formal acceptance criterion. Certified reference material use was explicitly reported in only 2 % of studies, whereas a reference method or independent procedure to establish or verify the target or assigned value was identified in 62 %. Across all included studies, 92 % reported errors, 74 % distinguished between error types, and 33 % considered differences in error consequences. Only 6 % explicitly weighted errors by severity or used risk-based prioritization (e.g., assigning greater weight to false-negative than false-positive results). No single diagnostic field consistently showed a more developed approach to incorporating errors into performance evaluation; instead, examples across fields illustrated approaches that considered how results are used, what consequences errors may have, how results are interpreted, and what actions follow. Overall, qualitative PT/EQA evaluation remains highly variable across studies. Clearer reporting of expected results, acceptance criteria, statistical methods, and error consequences may improve comparability and help laboratories interpret and use PT/EQA results for quality improvement.