Evaluation of internal control measurements in radioassay

The internal quality control measurements a laboratory makes are repetitive measurements monitored to see whether their variability and conformance to known values are within acceptable limits. These measurements provide a reasonably large volume of quality control information quickly. Evaluation of measurement quality requires pre-established analytical criteria. The most important of these criteria is the acceptable standard deviation of each type of measurement. This paper presents a method for calculating acceptable standard deviations, as a function of radionuclide content, by combining counting statistics and manipulation/calibration error. It includes the treatment of blank variability when it is beyond random variability. Control charts are used as the primary way of presenting data for evaluation. The paper also presents criteria for judging the acceptability of a set of replicate measurements, based on the number that are outside present levels, such as two-sigma levels. In addition, a procedure for applying chi-square tests to data accumulated over a period of time is given.