Importance of using soluble derivatives in assessing bilirubin interference: the example of gamma-glutamyltransferase

In assessing possible in vitro interference from bilirubin on analytical methods, different results are to be expected from using either unconjugated (uB) or conjugated (cB) bilirubin as test materials, because of their different solubilities. In vitro interference of a synthetic soluble bilirubin derivative (ditauro-bilirubin, dtB) on gamma-glutamyltransferase activity measurement has been studied, in comparison with uB. In three out of five analytical methods/systems for the measurement of the enzyme activity, significantly higher (negative) interference was observed in the presence of the soluble derivative. Whichever the mechanism for the observed effect, the opportuneness of using soluble derivatives in order to assess bilirubin interference is pointed out: pathological serum specimens, submitted for laboratory investigations, are indeed frequently loaded with soluble bilirubin conjugates.