Measurement and clinical usefulness of bilirubin in liver disease

Abstract

Elevated plasma bilirubin levels are a frequent clinical finding. It can be secondary to alterations in any stage of its metabolism: (a) excess bilirubin production (i.e., pathologic hemolysis); (b) impaired liver uptake, with elevation of indirect bilirubin; (c) impaired conjugation, prompted by a defect in the UDP-glucuronosyltransferase; and (d) bile clearance defect, with elevation of direct bilirubin secondary to defects in clearance proteins, or inability of the bile to reach the small bowel through bile ducts. A liver lesion of any cause reduces hepatocyte cell number and may impair the uptake of indirect bilirubin from plasma and diminish direct bilirubin transport and clearance through the bile ducts. Various analytical methods are currently available for measuring bilirubin and its metabolites in serum, urine and feces. Serum bilirubin is determined by (1) diazo transfer reaction, currently, the gold-standard; (2) high-performance liquid chromatography (HPLC); (3) oxidative, enzymatic, and chemical methods; (4) direct spectrophotometry; and (5) transcutaneous methods. Although bilirubin is a well-established marker of liver function, it does not always identify a lesion in this organ. Therefore, for accurate diagnosis, alterations in bilirubin concentrations should be assessed in relation to patient anamnesis, the degree of the alteration, and the pattern of concurrent biochemical alterations.

Keywords: bilirubin; biomarker; cholestasis; diazo method; liver disease.

Figure 1:

Bilirubin, isomers, and derivatives. (A) Linear or open configuration. (B) Closed configuration, ridge-tiled or mosaic. (C) Bilirubin diglucuronide (conjugated bilirubin). (D) Colored azopyrroles (azopigments) resulting from diazo reaction.

Figure 2:

Second-generation assays.Since in the past, they were thought to exclusively measure biologically active 1–84 PTH (7–84 PTH had not yet been identified), second generation assays are incorrectly known as “intact PTH assays”, but they have cross-reactivity with large C-terminal fragments 7–84. As these large fragments, with antagonistic actions to 1–84 PTH, are cleared by the kidneys, their/its percentage increases as glomerular filtration decreases renally. This condition must be taken into account when intact PTH is measured in CKD patients. ^aThis assay, no longer used, was validated by bone histomorphometry and was the assay of reference in most nephrology guidelines. ^bIsotopic IRMA assay. ^cIntact PTH assay with a different configuration than the other automated CLIAs. It shows interference with amino-PTH, since its amino-terminal antibody (curiously, the capture antibody) is very distal.

Figure 3:

Bilirubin metabolism and recirculation.