Calibration of direct reading photometers for the determination of neonatal bilirubin.

Blijenberg et al. (J. Clin. Chem. Clin. Biochem. 25, 737-741 (1987) (1)) proposed that the assigned values of calibrators for the determination of bilirubin with simple direct reading photometers (bilirubinometers or "bilimeters") should be determined on the same type of instrument. For this purpose, the instrument is calibrated with neonatal serum, whose bilirubin concentration is determined with a reference method unbiased by matrix effects. In the present study, this procedure was developed and tested. Bilirubin was determined in capillary plasma (n = 30) and venous serum (n = 31) of newborns. Measurements were made by direct spectrophotometry in 4 bilimeters, and by a micromodification of Vink's spectrophotometric method after dilution with caffeine reagent (Vink et al. Clin. Chem. 34, 67-70 (1988) (2)). When the bilimeters were calibrated with a commercial control serum, using the concentrations quoted by the manufacturer, the results were higher by 13% (plasma) and 19% (serum) than those obtained by Vink's method. These discrepancies practically disappeared when the bilimeter results were recalculated using a calibrator concentration determined according to Blijenberg. Application of the proposed method to other control sera gave assigned values up to 23% lower than those stated by the manufacturers. After calibration of the bilimeters with various neonatal sera (specimens drawn for therapeutic purposes and residues, obtained from the clinical laboratory), differences of about 10% were observed between the assigned values. It is therefore essential to use several sera. Minor differences were found between bilimeters.(ABSTRACT TRUNCATED AT 250 WORDS)