Letter to the Editor: Unlikeliness Limits Estimation

One of the requirements concerning the working instructions for measurement procedures (or measuring systems) given in the subclause 5.5.3 of the standard ISO 15189:2007 (1) states that the clinical laboratory documentation should include the reportable interval of examination results, and in the subclause B.5.6 of the Annex B states that, for each examination procedure, a range [interval] of values should be predefined to detect absurd or impossible results. Thus, any laboratory seeking accreditation for compliance with the mentioned standard shall establish the limits for these intervals. On the other hand, the same standard, in the subclause 5.7.1, states that authorized personnel shall review systematically the results of examinations in the post‐examination phase. One way of performing this systematic review is the plausibility control (2), using alert limits, among other tools, in order to detect doubtful results. Some of these doubtful results detected using this way could be results with a very low probability of belonging to the patient, here named unlikely results. Bearing in mind these two objectives, it is advisable to set a criterion to estimate unlikeliness limits which will define the unlikely results, aside from the clinical implication of these results. As there are not scientifically rigorous procedures to set unlikely limits, the establishment of such limits will be more or less arbitrary. The current text discusses several strategies that can be used to estimate the unlikeliness limits as well as the problems appeared in this estimation. The manuscript describes and compares different methods for the definition of the reportable interval of an examination procedure. In order to establish the unlikeliness limits of several biological quantities measured in the clinical laboratory of the Hospital Universitari de Bellvitge (Table 1), for each quantity, the measured values reported during years 2006 and 2009, and stored in the laboratory information system Omega 3000 (Roche Diagnostics España S.L., Sant Cugat del Vallès, Catalonia, Spain) were used, as long as 10 000 data or more were available. Among the different procedures proposed to estimate the unlikeliness limits, it is necessary to find out which one, despite of its arbitrariness, allows the establishment of these limits taking care that the number of unlikely results be reasonable under a professional point of view. One of the proposed procedures to get a limit is based in the estimation of fractiles beyond which will be very unlikely to find a result, although the choice of this fractile …