BilirubinTotal bilirubin
The yellow pigment from recycled red blood cells, processed by the liver.
Bilirubin is what remains when the body dismantles old red blood cells. It travels to the liver, which chemically modifies it so it can be excreted in bile. It is the pigment that makes bruises yellow as they fade and skin yellow in jaundice.
It matters because a raised level points to one of three things: red cells being broken down unusually fast, a liver not processing it, or bile not draining. Which of the three is distinguished by splitting the total into its direct and indirect parts.
In practice a mildly raised total bilirubin with otherwise normal liver tests is often Gilbert's syndrome, a common and harmless inherited variation in the processing enzyme, which fasting and illness make more pronounced.