Control charts for identifying systematic errors using control sera to detect antibody to Salmonella in an indirect ELISA

This study evaluated the preparation of Shewhart's control charts using the concept of rational subgroups for monitoring the Salmonella antibody ELISA used for surveillance of Danish pig herds. Control charts were prepared for a buffer control sample, a negative serum sample and a positive serum sample. The quality control variables were the natural logarithm (In) of the uncalibrated optical density (OD) for the buffer control sample and the negative serum sample, and the calibrated OD (OD%) for the positive serum sample. Testing round (run) within laboratory robot was chosen as the subgroup, and separate control charts were prepared for five robots. The control limits were set at six times the standard deviation for In(OD) and three times the standard deviation for OD%. Evaluation based on a number of sensitising rules for control charts produced from historical data showed that use of the control charts could reveal systematic analytical errors.