Conjugated bilirubin

Every day your spleen clears old red blood cells, and from the haemoglobin in them bilirubin arises, the yellow waste pigment. In that first form it does not dissolve in water and travels bound to albumin to your liver. There an enzyme attaches sugar groups to it, which is called conjugating, and with that it becomes water-soluble and ready for removal: through the bile to the gut, where it colours your stool brown, and with an excess through the kidneys into the urine, which then turns dark. Conjugated bilirubin is that processed part. In a healthy adult it is virtually zero, because the liver removes it directly and it does not linger in the blood. That makes it a sharp number: the unconjugated part rises in everyone after a night of fasting and in the inborn Gilbert variant, but the conjugated part only rises when drainage stalls. That is why it is measured as soon as total bilirubin is raised: it splits the question into production or processing on one side and drainage on the other. Your report prints it in µmol/L, sometimes as direct bilirubin, which is the same measurement. Normal in an adult is virtually zero: the upper limit lies between 3.4 and 5 µmol/L, depending on your lab, and on such a small number that difference matters. There is no lower limit, because low is the intention.

The split between processed and unprocessed tells something the total cannot: on which side of the liver it gets stuck. If mainly the unprocessed part is high, the question is about production, after accelerated breakdown of red blood cells, or about the processing step itself, as in Gilbert's syndrome, and that is almost always harmless. If precisely the processed part is high, your liver has done its work and it is not getting away afterwards, and then the question is about drainage through the bile. That is why this number counts. A gallstone blocking the drainage, a liver so under strain from alcohol, fat, a virus or a medicine that the tiny bile ducts inside it are squeezed shut, or a narrowing further along: all of them let the conjugated part rise, often together with ALP and gamma-GT, and with dark urine and pale stool when the pigment no longer reaches the gut. The honest limitation is that the number almost never stands out first. In a healthy adult it is virtually zero and it is usually only measured once the total is already raised. A low number says nothing, and on a small number the upper limit of your lab makes the difference between normal and raised. Always read it beside total bilirubin, because the ratio between the two carries the meaning.

A low conjugated bilirubin is naturally low and the healthy outcome: your liver processes the pigment and removes it straight away, and nothing lingers in the blood. There is no lower limit and no deficiency exists; a low number never needs explaining. A high conjugated bilirubin means drainage is stalling. In order of how often it occurs: blocked bile flow, through a gallstone blocking the drainage or a narrowing of the bile ducts, usually with a high ALP and gamma-GT and with dark urine; liver strain, where a liver swollen by alcohol, fat, a virus or a medicine squeezes the tiny bile ducts inside it shut, usually with a high ALT; and medication, such as some antibiotics, anabolic steroids and the pill, which inhibit the removal in the liver cell itself. In newborns other limits apply; those are in the table on this page and are not meant for you. Always read conjugated bilirubin beside total bilirubin, because the ratio between the two carries the meaning, beside ALP and gamma-GT, which rise with blocked bile flow, beside ALT, which rises with a strained liver, and beside the unconjugated part, which in Gilbert and after fasting lifts the total without anything being wrong.