Qualitative Test for Bile Pigments
and Urobilinogen in Urine^31
31.1 Theory
Bile pigments are breakdown products of the blood pigment hemoglobin that are
excreted in bile. Bilirubin (orange or yellow in color) and its oxidized form biliverdin
(green) are two important bile pigments. Bilirubin is formed by the degradation of
heme in reticuloendothelial system. After formation of unconjugated bilirubin,
albumin transports it to the liver where it conjugates with glucuronic acid to form
bilirubin diglucuronide. The conjugated bilirubin is excreted through bile. Unconju-
gated bilirubin is water insoluble, so it is unable to pass through the glomerulus and
is therefore not found in urine, whereas conjugated bilirubin is water soluble and is
filtered in the glomerulus and subsequently reabsorbed by the renal tubules; hence
normally bile pigments are absent in urine. Bilirubin glucuronides are hydrolyzed by
intestinalbacterialenzymeβ-glucuronidases to liberate bilirubin which is converted
to a colorless compound“urobilinogen.”A portion of the urobilinogen formed is
reabsorbed by enterohepatic circulation. Urobilinogen can be converted to a yellow-
colored compound“urobilin”in the kidneys and excreted. The major part of intesti-
nal urobilinogen is reduced to stercobilin by bacterial action which imparts brown
color to feces and is excreted (Fig.31.1).
31.2 Tests for Bile Pigments in Urine
31.2.1 Fouchet’s Test
31.2.1.1 Principle
The sulfate radicals present in urine react with BaCl 2 to form precipitates of barium
sulfate.Bilirubin present in the urine gets absorbed on the precipitates, and hence it
get conc. on treatment with Fouchet’s reagent. FeCl 3 in the presence of TCA oxidize
the bilirubin to biliverdin and produce blue or green color.
#Springer Nature Singapore Pte Ltd. 2018
V. Kumar, K. D. Gill,Basic Concepts in Clinical Biochemistry: A Practical Guide,
https://doi.org/10.1007/978-981-10-8186-6_31
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