Auditing quality control procedures in a chemical pathology laboratory--a multiple regression analysis

We undertook a retrospective analysis of the monthly test rejection rates and the monthly external quality assessment scheme performance indices for our laboratory's two automated analysers, and examined the association of these variables with measures of laboratory workload, manpower, staff training, instrument servicing, seasonal and temporal factors and changes of calibration, method and assigned internal quality control values. Using multiple linear regression and stepwise multiple linear regression, we found that test rejection rates differed significantly between instruments, and were highest on the instrument performing the widest variety and lowest volume of tests. On that instrument, rejection rates were significantly associated with the introduction of new staff and laboratory manpower levels, and also showed a highly significant trend upwards over the study period, independent of the effects of the other variables examined. External quality assessment scheme performance indices showed small trends over the study period. They were not related to the test rejection rates on either analyser but also showed a significant association with the introduction of new staff and a small but significant association with laboratory workload. We conclude that the training and introduction of new staff and decreased laboratory manpower levels may significantly increase the level of test rejection, and adherence to appropriate quality control protocols effectively maintains the quality of the laboratory's results, but may not be completely successful in filtering out the effects of some assignable causes of variation in test results. It is suggested that clinical laboratories use the statistical approach adopted here to identify factors which may be adversely affecting quality performance and running costs and to provide evidence that quality control procedures are both cost- and quality-effective.