A directly reacting fraction of bilirubin that is probably covalently bound to albumin (albumin-bound bilirubin) has recently been described. To determine its clinical importance we used a new high-performance liquid-chromatography technique to measure it in the serum of 200 patients with hyperbilirubinemia from various causes. Albumin-bound bilirubin was an important fraction (8 to 90 per cent) of total bilirubin in patients with hepatocellular and cholestatic jaundice as well as in patients with the Dubin-Johnson syndrome. It was not detected in normal volunteers, neonates with physiologic jaundice, or patients with Gilbert's disease or hemolysis. Thus, albumin-bound bilirubin appears in serum when hepatic excretion of conjugated bilirubin is impaired. It becomes a larger component of serum bilirubin as jaundice subsides, delaying resolution of this disorder and causing bilirubin to persist in plasma after it has disappeared from the urine.

We used two coated thin films to measure the concentrations of unconjugated, conjugated, and total bilirubin as well as bilirubin covalently bound to albumin ("delta" bilirubin) in more than 400 serum samples. We measured the unconjugated and conjugated species by determining their reflection densities at two wavelengths (400 and 460 nm) on a coating designed for the enhanced spectral measurement of bilirubin but which does not register the delta form. Total bilirubin was measured by use of a diazo-based thin film (Clin Chem 29: 37-41, 1983). We estimated the concentration of delta bilirubin by subtracting the sum of unconjugated and conjugated bilirubin from the concentration of total bilirubin. All measurements agree well with those by comparative methods, as shown by linear regression. Slopes ranged from 0.92 to 1.02, correlation coefficients from 0.935 and 0.998. Linear combinations of these values can also be used to compute other results; e.g., the sum of conjugated and delta bilirubin can be considered to be an estimate of "direct"-reacting bilirubin.

We compared results for total bilirubin as measured on a coated thin film and by the Evelyn-Malloy and Jendrassik-Gróf methods. We examined serum samples from patients and studied the effects of protein, hemoglobin, and lipids on bilirubin measurement. Results from the thin-film assay compared favorably with those of the other methods. Total and within-day precision (CV), assessed over a one-year period, were better than 6% and 3%, respectively, at all concentrations. Analytical recovery was 99 +/- 3%. Samples from individuals having a wide range of liver diseases demonstrated, by linear regression, good correlation between the thin-film method and the two wet-chemistry methods (correlation coefficients of 0.990 and 0.994). We conclude that the thin-film method offers a valid alternative assay for total bilirubin.