Why do we measure bilirubin in neonates? As most of you know, the natural physiological process of neonatal jaundice can be harmful to the newborn if the bilirubin concentration is too high. In the case of neonatal hyperbilirubinemia, the increase is primarily due to an immature liver which is unable to conjugate bilirubin due to decreased glucuronyl transferase activity. Consequently, if left untreated, infants can progress to severe bilirubin encephalopathy and kernicterus, a form of brain damage due to deposition of bilirubin in the basal ganglia and brainstem nuclei. There are several other causes that also need to be considered for hyperbilirubinemia, specifically hemolytic disorders. Practice guidelines have been created to assist with this clinical situation and have recommended that interpretation of serum total bilirubin measurements must consider the age of the infant in hours in addition to other risk factors such as gestational age. Currently, our clinicians have been educated to carry a nomogram in their pocket for proper determination. Perhaps in the future, if our LIS system can accommodate nomogram determinations, we will provide an interpretation with the result.
Several laboratory methods exist for measuring bilirubin. Most automated assays determine total, direct and indirect (calculated) bilirubin. Only the Vitros methods provide total, unconjugated, and conjugated bilirubin results. Calibration of these methods is complicated by the use of bovine serum, instead of human serum, enriched with unconjugated bilirubin and ditauraobilirubin. For diazo based methods, unconjugated bilirubin in bovine serum reacts incompletely and unpredictably. Therefore it is virtually impossible to assign accurate bilirubin calibrators using bovine serum as a protein base! To add to the problem, the reactivity of ditaurobilirubin can lead to underestimation of bilirubin concentrations. For better or worse, my laboratory uses the Vitros methods and we offer both the total bilirubin and neonatal bilirubin assays. In an ideal situation, I would prefer to only offer the neonatal bilirubin method to provide determinations on the unconjugated and conjugated bilirubin fractions.
Noninvasive or transcutaneous methods are also available for measuring bilirubin in newborns. This method uses reflectance densitometry and is very appealing as no blood draw is required. But how do they compare to serum measurements? The literature has several examples in which method comparisons between transcutaneous and automated clinical methods and the results indicate that transcutaneous results can be higher AND lower depending on the study. One study did link their lab analyzers results to the reference method procedure and showed that the transcutaneous method resulted in higher bilirubin results. Can the observed differences be explained by the problems associated with calibrators? Are the differences due to the operational use of the transcutaneous device? Could the differences be a combination of both calibrators and device? Maybe it’s something completely different. Nevertheless, many laboratories have done this comparison and implemented the device with a cutoff, above which a confirmatory measurement using a serum total bilirubin is requested. What has been your experience?