Chapter 043. Jaundice (Part 2)

Measurement of Serum BilirubinThe terms direct- and indirect-reacting bilirubin are based on the original van den Bergh reaction. This assay, or a variation of it, is still used in most clinical chemistry laboratories to determine the serum bilirubin level. In this assay, bilirubin is exposed to diazotized sulfanilic acid, splitting into two relatively stable dipyrrylmethene azopigments that absorb maximally at 540 nm, allowing for photometric analysis. The direct fraction is that which reacts with diazotized sulfanilic acid in the absence of an accelerator substance such as alcohol. The direct fraction provides an approximate determination of the conjugated bilirubin in...
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Chapter 043. Jaundice (Part 2) Chapter 043. Jaundice (Part 2) Measurement of Serum Bilirubin The terms direct- and indirect-reacting bilirubin are based on the originalvan den Bergh reaction. This assay, or a variation of it, is still used in most clinicalchemistry laboratories to determine the serum bilirubin level. In this assay,bilirubin is exposed to diazotized sulfanilic acid, splitting into two relatively stabledipyrrylmethene azopigments that absorb maximally at 540 nm, allowing forphotometric analysis. The direct fraction is that which reacts with diazotizedsulfanilic acid in the absence of an accelerator substance such as alcohol. Thedirect fraction provides an approximate determination of the conjugated bilirubinin serum. The total serum bilirubin is the amount that reacts after the addition ofalcohol. The indirect fraction is the difference between the total and the directbilirubin and provides an estimate of the unconjugated bilirubin in serum. With the van den Bergh method, the normal serum bilirubin concentrationusually is 17 µmol/L (virtue of its tight binding to albumin, the clearance rate of albumin-bound bilirubinfrom serum approximates the half-life of albumin, 12–14 days, rather than theshort half-life of bilirubin, about 4 h. The prolonged half-life of albumin-bound conjugated bilirubin explains twopreviously unexplained enigmas in jaundiced patients with liver disease: (1) thatsome patients with conjugated hyperbilirubinemia do not exhibit bilirubinuriaduring the recovery phase of their disease because the bilirubin is covalentlybound to albumin and therefore not filtered by the renal glomeruli, and (2) that theelevated serum bilirubin level declines more slowly than expected in some patientswho otherwise appear to be recovering satisfactorily. Late in the recovery phase ofhepatobiliary disorders, all the conjugated bilirubin may be in the albumin-linkedform. Its value in serum falls slowly because of the long half-life of albumin. Measurement of Urine Bilirubin Unconjugated bilirubin is always bound to albumin in the serum, is notfiltered by the kidney, and is not found in the urine. Conjugated bilirubin is filteredat the glomerulus and the majority is reabsorbed by the proximal tubules; a smallfraction is excreted in the urine. Any bilirubin found in the urine is conjugatedbilirubin. The presence of bilirubinuria implies the presence of liver disease. Aurine dipstick test (Ictotest) gives the same information as fractionation of theserum bilirubin. This test is very accurate. A false-negative test is possible inpatients with prolonged cholestasis due to the predominance of conjugatedbilirubin covalently bound to albumin. Approach to the Patient: Bilirubin The bilirubin present in serum represents a balance between input fromproduction of bilirubin and hepatic/biliary removal of the pigment.Hyperbilirubinemia may result from (1) overproduction of bilirubin; (2) impaireduptake, conjugation, or excretion of bilirubin; or (3) regurgitation of unconjugatedor conjugated bilirubin from damaged hepatocytes or bile ducts. An increase inunconjugated bilirubin in serum results from either overproduction, impairment ofuptake, or conjugation of bilirubin. An increase in conjugated bilirubin is due todecreased excretion into the bile ductules or backward leakage of the pigment. Theinitial steps in evaluating the patient with jaundice are to determine (1) whetherthe hyperbilirubinemia is predominantly conjugated or unconjugated in nature, and(2) whether other biochemical liver tests are abnormal. The thoughtfulinterpretation of limited data will allow for a rational evaluation of the patient(Fig. 43-1). This discussion will focus solely on the evaluation of the adult patientwith jaundice. Figure 43-1