CALIBRATORS AND CONTROL SAMPLES FOR BILIRUBINOMETERS

The different matrix properties of neonatal serum and commercial contr ol samples can lead to considerable errors in the calibration and cont rol of bilirubinometers. These difficulties can be avoided by calibrat ion with serum from healthy adults which is supplemented with unconjug ated bilirubin. But this procedure is impracticable for most routine l aboratories. Under certain preconditions, control samples, with biliru bin concentrations determined with correctly calibrated bilirubinomete rs or spectrophotometers, are also suitable as calibrators. This was e stablished by determination of the bilirubin concentration of 16 diffe rent control samples, using both the reference method and correctly ca librated bilirubinometers or spectrophotometers in three or four speci alist laboratories. This was also confirmed in several interlaboratory surveys, some involving up to 72 laboratories. The results of these i nvestigations show that a control sample should be used for the calibr ation of a bilirubinometer only if it meets the following precondition s: 1. There should be no significant difference between the bilirubin values determined with the reference method and with a correctly calib rated spectrophotometer or bilirubinometer. 2. The bilirubin concentra tion should lie in the range 230 - 300 mumol/l. The photometric respon se of bilirubinometers has a limited linear range, so that analytical results greater than 300 mumol/l must be rated as basically unreliable .