Evaluation of interlaboratory proficiency surveys of bilirubin determinations in sera of newborns.

We evaluated the results of two surveys of neonatal bilirubin determinations in 66 Slovakian clinical chemistry laboratories. Identical control samples prepared as described by Vink (Clin. Chem. 33 (1987) 1817-1821) and commercially available control serum were used for both surveys. These control samples were analysed in the participating laboratories using various modifications of diazo techniques based on the Grof-Jendrassik principle, and direct two-wavelength spectrophotometry according to Vink (Clin. Chem. 34 (1988) 67-70). The interlaboratory precision and accuracy of these two different analytical principles were compared. For diazo techniques the CVs varied from +/- 6.8% to +/- 10.3%, while the CVs for direct spectrophotometry were between +/- 5.3% and +/- 6.4%. Better results were obtained by recalculating the raw data of the analyses from all laboratories, using the reported results for the analysis of an identical calibrant. Accuracy was evaluated as the percentage of acceptable results (within +/- 7% of the reference values). About 60% of the results obtained with diazo methods were considered acceptable, compared with about 22% of those obtained by direct spectrophotometry. However, results recalculated on the basis of the identical calibrant showed greater accuracy; acceptable results then varied between 74% and 82% of the total for the diazo techniques and from 83% to 91% for direct spectrophotometry.