Unconjugated bilirubin

Every day your spleen clears old red blood cells, and from the haemoglobin in them bilirubin is released, the yellow waste pigment. In that first form it does not dissolve in water; it travels bound to albumin to your liver, where an enzyme attaches sugar groups to it, which is called conjugating. Only then can it go through the bile into your gut. Everything that has not yet had that processing is called unconjugated bilirubin, and that is normally the largest part of what circulates in your blood. The part is not measured but calculated: total bilirubin minus conjugated bilirubin, both from the same tube. It is the fraction that holds the most common explanation for a slightly raised bilirubin. In Gilbert's syndrome, an inborn variant found in about one in twenty people, the enzyme that attaches the sugar groups works at half strength, and precisely this part accumulates while the conjugated part stays neatly low. The same happens in everyone after a night of fasting, poor sleep or heavy exertion, because then the enzyme gets less fuel. Your report prints it in µmol/L, sometimes as indirect bilirubin, which is the same; otherwise you calculate it with the calculator on this page. There is no band, because it is a subtraction. What you do have are the bands of the two values beneath it: 19 to 24 µmol/L as the upper limit for the total and 3.4 to 5 for the conjugated part, depending on your lab. A low number is naturally low and says nothing.

This is the part that answers whether a raised bilirubin means anything. If mainly the unconjugated part is high while the conjugated part stays low and the liver values are normal, the explanation is almost always Gilbert's syndrome or a night of fasting, and then nothing is wrong. Anyone who knows that once does not need to be alarmed again at every slight rise. The second reason is that the part says something about the balance between breakdown and processing. If it rises without Gilbert and without fasting, more red blood cells are being broken down than the liver can keep up with, and then haemoglobin, reticulocytes and LDH belong with it. That is rare, but it is why the part is always looked at separately when the total is raised. The honest limitation is that it is a subtraction and not a measurement, without a band of its own. It moves with everything that moves the total and the conjugated part, and it is the part fasting lifts, so a fasting draw makes it come out higher than an ordinary one. Always read it beside the total and the conjugated part, because the ratio between the two carries the meaning.

A low unconjugated bilirubin is naturally low and says nothing: your spleen breaks down red blood cells at an ordinary pace and your liver keeps up. There is no lower limit and no deficiency exists. A high unconjugated bilirubin means more production or slower processing. From common to rare: Gilbert's syndrome, the harmless inborn variant in which the enzyme works at half strength and total bilirubin is slightly raised with otherwise normal liver values; fasting or a night of poor sleep, because then the enzyme gets less fuel and this part rises in everyone, even without Gilbert; liver strain, through alcohol, fat, a virus or a medicine, where the conjugated part and ALT also rise; and rarely accelerated breakdown of red blood cells, where haemoglobin falls and reticulocytes and LDH rise. Set unconjugated bilirubin beside the total and the conjugated part, because the ratio carries the meaning, beside ALT and gamma-GT, which show whether the liver is strained, and beside haemoglobin and reticulocytes if the part rises without Gilbert and without fasting.