About reporting and evaluating results
What standard deviation for proficiency assessment (SDPA) is used in microbiology PT schemes? There are many sources of variation in microbiological testing and the SDPA used to assess performance therefore needs to be fit-for-purpose and take all possible sources of variation into account. From experience and historical data, AXIO Proficiency Testing uses a fixed SDPA value of 0.35 log10 for the majority of microbiological tests. How do I report a ‘presumptive’ result in microbiology? Report your result as usual but record in the comments section that the result is ‘Presumptive’. What is the purpose of scoring my result? Once the assigned value for the parameters under test has been established, participant laboratories are assessed on the difference between their result and the assigned value, with this difference being represented by a performance score called a z score. This provides a simple and consistent measure of performance which is the key to monitoring competence and implementing an improvement programme as required. How is a z score calculated? The participant’s result, x, is converted into a performance score (z score) using the following formula: z = (x - X) SDPA Where: X = the assigned value SDPA = Standard Deviation for Proficiency Assessment For small data sets, there will be increased uncertainty around the assigned value if derived from a consensus value from participants’ results. In such cases, performance scores may not be provided, or may be given for information only. The z score expresses performance in relation to the assigned value and the standard deviation for proficiency assessment (SDPA). A z score of 2 represents a result that is a distance of 2 x SDPA from the assigned value.
How do I interpret my results? For quantitative examinations, participant performance is assessed using the z score, and the following interpretation is given to results: |z| ≤ 2.00 Satisfactory result 2.00 < |z| < 3.00 Questionable result |z| ≥ 3.00 Unsatisfactory result For qualitative examinations or semi-qualitative results, laboratories reporting the assigned result or range of results will be considered correct, and therefore have satisfactory performance. What are the advantages of using a z score to assess performance? • Results can be expressed in a form that is easy to interpret and understand. • Results can be summarised in graphical or tabular form to depict overall performance. • A z score allows participants to directly compare their own result with others. • If consistent statistical values are applied, a z score enables participants to monitor and trend their own performance over time. It is important to interpret any performance score in the full context of the overall results and in the context of a laboratory’s own quality control measures.
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