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Bilirubin

Synonym: haematoidin
German: Bilirubin

1 Definition

Bilirubin is the yellow-brownish degradation product of hemoglobin. In blood serum about 90 per cent of bilirubin exist as primary bilirubin bound to albumin.

2 Physiology

Bilirubin is generated during the degradation of hemoglobin which is mainly released during the degradation of old red blood cells by macrophages in the spleen and liver. The heme group of the hemoglobin is transformed by the enzyme heme oxygenase into the intermediate biliverdin which in turn is metabolized into bilirubin by the biliverdin reductase enzyme. This biliverdin immediately binds to albumin in the blood plasma.

After being taken into a hepatocyte, biliverdin binds to glucuronic acid. The resulting product is conjugated bilirubin.

3 Classification

Due to the metabolic pathways explained above, different forms of bilirubin can be distinguished:

Clinical analyses usually measure total serum bilirubin (TSB) and direct bilirubin. The amount of indirect bilirubin can be calculated as the difference of the other two values.

Bilirubin can be broken down by shortwave light. This is the reason why samples intended for bilirubin analysis have to be guarded from light (preanalytics).

4 Pathophysiology

The development and excretion of bilirubin can be disrupted due to several conditions, e.g. Gilbert's syndrome, Crigler–Najjar syndrome or Rotor syndrome. Due to bilirubin being a degradation product of hemoglobin, increased serum concentration is a marker for increased hemolysis. Another cause for elevated bilirubin levels is vitamin B12 deficiency.

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