Bilirubin is a bile pigment that is formed as a result of the destruction of red blood cells. The blood cells under physiological conditions disintegrate 800 billion(!) cells every hour and the same amount is formed. The result is 2 components: haem and globin. Haem then turns into indirect (unconjugated, free) bilirubin, binds to albumin and is transported to the liver. There it is conjugated ('straightened'). Bilirubin now becomes straight (conjugated, bound). Hence the following three laboratory values:

Direct bilirubin + indirect bilirubin = total bilirubin.

Bilirubin gives the physiological excretions a yellow coloring in the urine and a dark brown coloring in the feces (respectively, the more pigment, the more intense the coloring).

A pigment increase above 30 nmol/l will cause the sclerae of the eyes to yellow, and at 40 nmol/l the skin to yellow. This clinical picture is most commonly seen in hepatitis (inflammation of the liver).

Physicians need this test to determine the cause of yellowing of the skin and to monitor the progression of liver disease.

High indirect bilirubin is caused by excessive destruction of erythrocytes (haemolytic anaemia), as large quantities are produced from dead blood cells. The skin colour will be citric (pale due to anaemia + yellowish due to bilirubin).

An increase in the straight fraction may be seen in hepatitis and stagnation of bile - the outflow into the intestine is disturbed and there is a large inflow into the bloodstream. The stool will become discoloured and the urine will become the colour of beer.

There are congenital diseases (Kriegler-Neillard, Gilbert, Dabin-Johnson) in which bilirubin metabolism is disturbed.