Efficacy of internal and external quality control in chemical pathology

Quality control in chemical pathology is based on internal and external quality assessment. The internal quality control in clinical chemistry section of AKUH laboratory is based on the analysis of commercially available Beckman norma! and abnormal range control sera. These have known concentra­tions of blood chemistry constituents. After repeated estimations, mean values of all constituents were established for both normal and abnormal sera. Patients samples were analysed only when both controls were within ± 2SD from the mean or one control reads within ± 2SD and the other read between ± 2—3SD (once only) for each constituent. Samples were not analysed if one control was greater than ± 3SD from the mean or both controls were greater than ± 2SD or one control was between ± 2—3 SD on two succes­sive runs. External quality control sera with unknown constituent concentrations were analysed twice monthly and results sent to Weilcome England. Our results were compared with the mean of estimations by 1000 laboratories for each constituent. The acceptable standard deviation from this mean for each constituent was ± 2SD. External quality control evaluated our accuracy in wide ranges of constituent concentrations encountered and helped us identify under or over reporting of differenit parameters, both substrates and enzymes, at high or low levels or vice versa which internal quality control could not address. Hence external quality control formed the basis of our corrective actions in terms of equipment and rea­gents. We conclude that internal quality control is limited in the sense that its efficacy has to be tested by external quality assessmen