Bilirubin

Bilirubin is a yellow-orange pigment produced when your body breaks down old red blood cells. Red cells live for around 120 days, and as they are retired, their haemoglobin is dismantled and the iron is recycled. What is left over becomes bilirubin. The liver picks it up from the blood, chemically modifies it so that it dissolves in bile, and sends it out into the digestive tract, where it contributes to the characteristic brown colour of stools. Measuring bilirubin tells you how well this whole recycling and clearance chain is working.

Bilirubin initially circulates in an "unconjugated" form, insoluble in water and carried attached to albumin. The liver converts it into a soluble "conjugated" form that can be excreted in bile. Bilirubin can therefore accumulate for three broad reasons: too much red cell breakdown (haemolysis), the liver struggling to process it, or a blockage downstream preventing bile from leaving the liver (gallstones, a stricture, or tumour in the bile ducts). When bilirubin builds up significantly, the pigment deposits in the skin and the whites of the eyes, causing the yellow tinge known as jaundice.

One thing worth knowing is that a mild, isolated rise in bilirubin is extremely common and is very often caused by Gilbert's syndrome. Gilbert's is a harmless inherited variation in which the liver enzyme that conjugates bilirubin works slightly less efficiently. It affects roughly 5% of adults, runs in families, and causes no symptoms other than occasional mild jaundice. Levels characteristically fluctuate and can rise during fasting, illness, dehydration, or poor sleep. Because Gilbert's is so common, a mildly raised bilirubin picked up incidentally, with completely normal ALT, AST, GGT, ALP and a normal full blood count, is almost always Gilbert's rather than liver disease.

The pattern across your other markers separates the causes. Raised bilirubin with raised ALT and AST suggests liver cell damage, raised with ALP and GGT suggests bile duct obstruction, raised with low haemoglobin, high LDH and low haptoglobin suggests haemolysis. If your bilirubin is raised, your GP will look at the whole picture to decide whether any further investigation is needed.