Bilirubin is produced in the human body during the breakdown of hemoglobin. Haemoglobin is the red blood pigment whose main function is to store oxygen in the blood cells. Human blood owes its red colour to it.

Bilirubin, on the other hand, is yellowish to brownish in colour and lipophilic, i.e. it is well soluble in fat but poorly soluble in water. As a breakdown product, bilirubin is excreted via the liver into the intestines and finally via the stool. As a laboratory value, bilirubin is therefore determined primarily in the diagnosis of diseases of the liver and bile ducts.

Bilirubin metabolism

Blood cells have a lifespan of about 120 days, after which they are broken down mainly in the spleen. Hemoglobin is released in the process. Haemoglobin consists of a protein component and the haem group, the actual red blood pigment.

The protein portion is metabolized in the body in different ways. Haem, on the other hand, is a ring-shaped molecule that requires its own metabolic pathway for its breakdown. First, the ring structure of heme is split by a special protein, hemoxygenase.

This produces the so-called biliverdin, which is greenish in colour. The second step is performed by another enzyme, the so-called biliverdin reductase. It converts biliverdin into the yellowish bilirubin.

Bilirubin is poorly soluble in water and must therefore be bound to special proteins such as albumin in the blood. This bilirubin is also called unconjugated or indirect bilirubin. The next step happens in the liver.

Here the bilirubin reaches the liver cells, which convert it into bilirubin diglucuronide through several intermediate steps. This is bilirubin to which glucuronic acid has been bound. This process improves the water solubility of the bilirubin and it can be excreted into the intestine via the bile ducts.

It is now called conjugated or direct bilirubin. You can also read more about this topic here: Function of the liver, tasks of the liverHowever, this is not quite the end of the bilirubin metabolism. In the intestine, bilirubin diglucuronide is further metabolised by bacteria.

From bilirubin they form stercobilin, for example, which is partly responsible for the brownish colour of the stool. In addition, a part of the excreted bilirubin is reabsorbed, so that a constant circulation between the intestine and the liver is created. Bile, bile duct, gall bladder

What does the bilirubin level say?

Bilirubin is formed when red blood cells die. A healthy and freely working liver and bile is necessary for its breakdown. Changes in these areas also result in altered bilirubin levels.

It is important to differentiate between indirect and direct bilirubin. Indirect bilirubin is further metabolised in the liver to direct bilirubin. Depending on which of the two values increases, the location of a possible damage can therefore be determined precisely.

The designations direct and indirect bilirubin are due to the different measuring methods used to determine the concentration. Similar to other blood parameters, the bilirubin concentration is determined in serum, i.e. the aqueous blood fraction. Normal values for indirect bilirubin here are below 1.0 mg/dl (17.1/l).

The concentration of direct bilirubin, however, is lower at less than 0.2 mg/dl (3.4 μmol/l). The total bilirubin concentration should therefore be below 1.2 mg/dl (20.5 μmol/l). These guide values may change depending on the measurement method and the respective laboratory.

Values that are too low do not occur in any known disease and therefore do not indicate any damage. Increased bilirubin values, on the other hand, can have various causes. If the bilirubin concentration in the blood rises very sharply, it can escape from the vessels into the surrounding tissue.

Since bilirubin has a typical yellowish colour, the respective tissue is also stained. This is often first seen on the conjunctiva of the eyes, which appear yellowish. If the bilirubin levels are more strongly increased, the entire rest of the body skin also appears yellow.

In addition, there is itching in the affected tissue. This is known as jaundice or icterus. An icterus can be divided into a prehepatic, intrahepatic and posthepatic form according to its cause.

The prehepatic form has its cause “before the liver” (pre – before, hepar – liver), the intrahepatic form is caused in the liver (intra – inside) and the posthepatic form is mostly caused in the bile following the liver (post – after, after). The cause of a prehepatic icterus can be, for example, a shortening of the life span of the erythrocytes. If it is below 50% of the norm (120 days), more indirect bilirubin is produced than the liver can convert to indirect bilirubin and excrete.

As a result, the indirect bilirubin and, of course, the total concentration are increased. A posthepatic icterus, on the other hand, is usually caused by a disruption of the bile flow. Indirect bilirubin is metabolized further to direct bilirubin as in healthy individuals, but the direct bilirubin can no longer leave the body and accumulates.

The consequence is jaundice with increased direct bilirubin. In intrahepatic icterus, both indirect and direct bilirubin may increase due to the key role of the liver in bilirubin metabolism. These topics may also be of interest to you: increased liver values, liver diseases, hepatitis, neonatal jaundice