Detection of short-term fluctuations in measurement systems by determination of extreme values and range

In the new draft, in preparation, of the guidelines of the German Medical Council (Richtlinien der Bundesärztekammer, RiLiBAK) for quality management of clinical laboratory measurement systems, particular emphasis is placed on the establishment of a lower and upper limit for each control value in a control cycle. In the present study, an attempt is made to show that this approach alone does not detect even relatively large short-term changes in a measurement system. A substantial improvement is possible with modest outlay, by also examining the difference between the largest and smallest individual control values, i.e. by determining the current range. Appropriate practical limits for this procedure are derived from the theoretical behaviour of extreme values, based initially on the usual assumption of a normal distribution. Furthermore, it is shown that these limits are also meaningful for other assumed appropriate distributions.