Assessment of the fundamental accuracy of the Jendrassik-Gróf total and direct bilirubin assays.

Using unconjugated bilirubin (Bu) and authentic human disconjugated bilirubin (dBc) supplemented in low-bilirubin serum pools, we have directly verified for the first time that the Jendrassik-Gróf total bilirubin assay modified after Doumas et al. (Clin. Chem. 19: 984-993, 1973) detects Bu and dBc quantitatively, in Bu-equivalent concentrations relative to gravimetric concentration assignment reinforced with quantification by nuclear magnetic resonance (Wu et al., Clin. Chem. 26: 1323-1335, 1980). By contrast, the Jendrassik-Gróf direct bilirubin assay (also modified after Doumas) quantifies only 70 +/- 5% of the gravimetrically determined dBc as Bu equivalents. By using Bu and dBc (instead of Bu only, as is routinely done) to calibrate the responses of the total and direct bilirubin assays, the analyses of Bu and dBc become much more quantitative. We mathematically explored the meanings of the conventional terms total, direct, and indirect bilirubins. The clinical and diagnostic implications of our observations are discussed.