ABSTRACT
Proficiency Testing (PT) External Quality Assessment (EQA) schemes are designed to ascertain the ability of individual laboratories to perform satisfactorily with respect to their peer laboratories or to limits imposed by external sources. Observed deviation of a laboratory result for a PT sample must be entirely attributed to the laboratory and not to the PT provider. To minimize the probability that deviations could be attributed to the PT provider, sample homogeneity should be assured. It is generally required that for quantitative parameters, the standard deviation among PT units should be calculated on the basis of duplicate measurements of at least 10 samples chosen at random, and the standard deviation among PT units should not exceed 0.3 times the standard deviation used to evaluate laboratories. Because this approach has important drawbacks, an alternative procedure is proposed by applying the theory of acceptance sampling to the assessment of sample heterogeneity for both quantitative and qualitative data and deriving acceptance limits on the basis of minimizing the probability of falsely evaluating laboratories. For obtaining acceptance limits for quantitative parameters, a distinction is made between laboratory evaluation using fixed limits on the one hand and laboratory evaluation using limits that are based on the variability of the reported results on the other hand. Sequential tests are proposed to evaluate sample heterogeneity by means of a comparison with the χ2 distribution. For qualitative parameters, acceptance-sampling plans are proposed that are based on minimizing the joint probability of rejecting batches that have a satisfactory amount of defective samples and accepting batches unnecessarily. The approach for quantitative parameters is applied on samples for a PT scheme of ethanol quantification and for qualitative parameters on the presence of monoblasts in a blood smear. It was found that five samples could already be enough to prove that the batch was homogeneous for quantitative parameters, although more than 20 samples were needed to prove homogeneity for qualitative parameters. This study describes a direct relation among the objective of an PT round, the criteria for evaluating the results, and the sample heterogeneity. When samples are effectively homogeneous, less measurements are needed than current practices require. A drawback of the proposed approach is that the number of samples to be tested is not known beforehand, and good knowledge of the analytical variability is crucial. The formulas to be applied are relatively simple. Despite the drawbacks, the proposed approach is generally applicable for both quantitative and qualitative data.
ABSTRACT
In Croatian medical laboratories (ML), external quality assessment (EQA) has a long tradition of almost half of a century. At national level, EQA is provided by Croatian Centre for Quality Assessment in Laboratory Medicine (CROQALM) which is a part of Croatian Society of Medical Biochemistry and Laboratory Medicine.This case study aimed to summarize the main challenges, which are set to CROQALM and their possible solutions.CROQALM has 10 schemes, covering majority of analysis for which medical biochemistry laboratories in Croatia are authorized for, including pre-analytical and post-analytical phase of laboratory work. Assessment scheme has three exercises per year. One sample per scheme and exercise is distributed to participants depending on their application. All data transfer and evaluation of the results are done using web interface and statistical software for evaluation of quality in laboratory medicine.Since CROQALM has relatively small number of participating laboratories (N = 197) with lot of different manufacturers of instruments used for analysis in all schemes, constant challenges are present in the evaluation of the results (commutability problems, statistical analysis etc.). Further, number of participating medical laboratories is even lower for highly specific parameters, which are in the scope of clinical laboratories only.Despite the obstacles we are faced to, EQA at national level is useful tool regarding standardization and harmonization aspects in total testing process within the country. Furthermore, it gives participating laboratories recognition and proof for meeting expected quality criteria in the community they serve.
