Scoring system to evaluate analytical performance of laboratories participating in an EQA scheme for hormones

To allow laboratories an easy evaluation of thei own analytical performance, a new scoring system has been adopted in the External Quality Assessment (EQA) scheme for hormones (Immunocheck EQA; 1300 participants in Italy and in France; 18 control samples/year; 16 analytes). The score is assigned both to the results of a single assay (assay score) and cumulatively to all results of samples assayed in a control cycle (cycle score). Assay score. Each result is scored according to its deviation from target value expressed in SD units (Z-value). In detail the scores are: 4 (excellent) if Z <0.5, 3 (good) if 0.53. Z-value is computed as ratio of percent deviation from target and "state-of-the-art" imprecision. Method mean is assumed as target; "state-of-the-art" imprecision is computed as mean CV of the methods most used in the survey (within-method, between-laboratories imprecision). Cycle score. The sum of score assigned to all samples assayed in a control cycle, normalised by the maximum achievable total score and expressed as tenth, describes the analytical performance off the laboratory. As an example, cycle score for fT4 assay was found better than 6/10 for 82% of participants, between 3/10 and 6/10 for 14% and worse than 3/10 for the remaining 4%.