The interlaboratorial quality assurance program for blood lead determination. An evaluation of methods and results

For over five years a national program, promoted by a working group of the Istituto Superiore di Sanità (Italian National Institute of Health), has been active in Italy for the quality control of the blood lead levels determination. The program is based on the adoption, by the laboratories, of the same known-titre materials, for the internal quality control, and on the participation in periodical collaborative exercises for the external quality evaluation. The promoting laboratory prepared the control samples, verified their homogeneity and stability and distributed them to the laboratories following a randomized procedure; then, it provided a preliminary elaboration of the results (precision, difference from the median, distribution) after each exercise, and carried out the global evaluation of the performances of each laboratory after at least one year of activity in the program using parametric (regression analysis) and non-parametric (evaluation of the results according to pre-determined acceptability criteria) statistical methods. After four years of activity, the results obtained show that the adopted scheme and the procedures used turned out to be adequate. The study of the regression parameters between the results of each laboratory and the medians of the results of all the laboratories has confirmed the validity of the graphic criterion adopted, also yielding specific information on the relative contribution of the different kinds of error (systematic, constant and/or proportional and casual) to the global error. Furthermore, the proportion of the laboratories with "good level" performances (i.e., acceptable results in at least 80% of the examined samples) has increased from approximately 30% in the first phase to approximately 50% in the fourth phase.