Total-calcium measurement in serum from neonates: limitations of current methods

In the routine analysis for calcium in serum from neonates we noted large discrepancies in icteric samples analyzed by several different methods. The present investigation was undertaken to demonstrate the magnitude of the problem of bilirubin interference in calcium analysis and to assess the specific applicability of commonly used micro-methods in the pediatric or neonatal laboratory. The intention is not to advocate any single methodology but rather to point out inadequacies that must be considered. Colorimetric and fluorometric methods correlated poorly with an atomic absorption spectroscopic method for samples from pediatric patients. In addition, we noted significant interference proportional to the concentration of bilirubin. A fluorescence titration method was shown to be unacceptable for pediatric use because of the variable and unpredictable response to bilirubin interference. The study emphasizes the need for thorough invesigation of methods before they are put to use in the specialized pediatric setting.