Calibration charts
Calibration charts are a new feature of MultiQC6 which must not be taken as an established method, but as a first step toward an alternative approach to the analysis of QC data.
In the fifties, Levey and Jennings based their interpretation of QC data on the very concentrations resulting from the assay of a set of two or three control materials. Reducing an analytical process to a set of discrete and independent quality characteristics is however conceptually unsatisfactory. It would be desirable to take into account the continuous nature of analytical processes. Thus QC in medical laboratories should take benefit from the recent research activity in the area of quality profiles which was reviewed by Woodall et al. (Using Control Charts to Monitor Process and Product Quality Profiles. Journal of Quality Technology 2004, 36: 309-20).
Profiles are useful when a quality variable is functionally dependent on one or more explanatory or independent variables. In our medical laboratory field, profiles are the response curves of analytical methods. They relate measured concentrations to true concentrations. The theoretical relationship is equality, a very basic linear profile.
Instead of dealing with each QC vector as a set of a low, a medium and a high concentration that are directly controlled, we use them to calculate the regression line that relates the three assayed values to the 3 target values. This relationship between the “output” (actual assayed concentration) and the “input” (true target concentration) provides us with an estimate of the instant calibration line which becomes the new QC object. The resulting chart is therefore named calibration chart. It controls the day-to-day variations of the instant calibration line by means of a single chart.