Bilirubin blood test: what it measures, normal levels, and what high bilirubin means

Red blood cells live about 120 days. When the spleen removes them, the heme in their hemoglobin is converted to bilirubin, which travels to the liver bound to albumin. This first form is unconjugated, or indirect, bilirubin; it is fat-soluble and cannot be excreted. The liver attaches a sugar group to it, producing conjugated, or direct, bilirubin, which is water-soluble and is secreted into bile, giving stool its brown color and, when it spills into urine, turning urine dark.

The routine test reports total bilirubin. When it is high, laboratories can fractionate it into direct and indirect portions, and the split is the key to interpretation: a predominantly indirect rise means too much bilirubin is being made or the liver cannot conjugate it; a predominantly direct rise means conjugated bilirubin cannot leave, because liver cells are damaged or the bile ducts are blocked. Skin and eyes turn visibly yellow, jaundice, when total bilirubin passes roughly 2.5 to 3 milligrams per deciliter.

Bilirubin is the yellow pigment left over when red cells are broken down, so a high bilirubin colors things:

The commonest cause of a mildly high bilirubin in an otherwise healthy adult is Gilbert's syndrome — a harmless inherited variant affecting several percent of people, in which unconjugated bilirubin rises with fasting, illness or stress and needs no treatment. A higher or rising bilirubin, or one that is mostly conjugated (direct), points to liver disease, bile-duct obstruction or rapid red-cell breakdown and needs evaluation.

A low bilirubin has no clinical meaning. Fasting before the draw raises bilirubin modestly, which is worth knowing when a mildly high result appears on a fasting panel.

Bilirubin is on every comprehensive metabolic panel and hepatic panel and so is checked routinely. It is ordered deliberately for jaundice, dark urine, pale stools or itching; in known liver disease to gauge how much function is impaired; in suspected hemolysis, when red cells are being destroyed; in gallstone disease and suspected bile duct obstruction; and in newborns, in whom bilirubin is monitored closely in the first days of life for a reason unrelated to adult disease.

A large share of people who look up their bilirubin do so because a routine panel showed a mild elevation with everything else normal. That pattern is usually Gilbert syndrome, described below, and understanding it saves a good deal of worry and testing.

Total bilirubin comes with the comprehensive metabolic panel and the hepatic function panel, and direct bilirubin can be added or ordered separately; your doctor can order any of these, or you can order them online through TestWell for a draw at a Quest or Labcorp patient service center, with a licensed physician reviewing the order and results in your portal within one to three business days. When bilirubin is the question, the hepatic panel plus direct bilirubin and a CBC covers the liver, duct and red cell causes in one draw.

Fasting is required for the glucose on a metabolic panel, and fasting itself matters here: in people with Gilbert syndrome, fasting, dehydration, illness, strenuous exercise and poor sleep all push bilirubin up, so a level drawn after an overnight fast during a cold can read higher than usual. Mention recent illness, alcohol, and medications; some antibiotics, HIV drugs and other medicines raise bilirubin, and a few laboratory interferences exist. Keep the sample out of bright light, which the laboratory handles, because light degrades bilirubin.

Most laboratories report a normal adult total bilirubin of about 0.1 to 1.2 milligrams per deciliter; some ranges extend to 1.0 or 1.4, and the range on your report applies. Direct bilirubin is normally below about 0.3; indirect is the difference and makes up most of the total in health. Men run slightly higher than women, and levels fall a little with age. Newborn ranges are entirely different and are interpreted by hour of life.

The direction of the split organizes the causes. A total bilirubin up to roughly 3 to 4 with almost all of it indirect, normal liver enzymes and a normal CBC is the signature of Gilbert syndrome. Indirect predominance with anemia, a high reticulocyte count, a high LDH and a low haptoglobin means hemolysis. Direct predominance with high ALT and AST means liver cell injury; direct predominance with a high alkaline phosphatase and GGT means the bile ducts, and imaging comes next.

Gilbert syndrome is the most common cause of a mildly high bilirubin in otherwise healthy adults, affecting perhaps 5 to 10 percent of the population. An inherited reduction in the enzyme that conjugates bilirubin lets the indirect form accumulate, especially with fasting, illness, stress, dehydration or exertion. It causes no liver damage, needs no treatment, and is diagnosed by the pattern: mild indirect elevation, normal liver enzymes, normal blood count, often known in relatives. Occasionally it produces a faint yellowing of the eyes during illness, which is harmless. Genetic testing exists but is rarely necessary.

Hemolysis, the early destruction of red cells, raises indirect bilirubin along with LDH and the reticulocyte count and lowers haptoglobin; causes include inherited red cell disorders such as sickle cell disease and hereditary spherocytosis, immune destruction, mechanical heart valves, some infections and medications. Liver cell injury from viral hepatitis, alcohol, drugs, fatty liver disease or cirrhosis raises both fractions with high ALT and AST. Bile duct obstruction from gallstones, strictures, pancreatitis or tumors of the pancreas or ducts raises the direct fraction with a high alkaline phosphatase, pale stools, dark urine and often itching; painless jaundice with weight loss in an older adult is urgent because of what it can signify.

Severity is judged by the company bilirubin keeps rather than the number alone. A total bilirubin of 2 with normal enzymes in a fasting healthy adult is a different situation from the same value with a high alkaline phosphatase and abdominal pain, which needs same-week imaging, or with confusion and easy bruising, which is an emergency.

A low bilirubin has no clinical significance. Very low values are seen with some medications, in smokers and in some healthy people, and research associating low bilirubin with cardiovascular risk reflects its antioxidant properties at the population level, not a condition to treat. No follow-up is needed.

Fractionate first: direct and indirect bilirubin, with a hepatic panel and a CBC on the same draw. Mild indirect elevation with normal enzymes and a normal blood count is Gilbert syndrome and needs no further testing beyond, if wanted, a repeat when well and fed. Indirect elevation with anemia calls for a reticulocyte count, LDH, haptoglobin and a blood smear. Direct elevation with high ALT and AST follows the liver-injury pathway of hepatitis testing, iron studies and ultrasound; direct elevation with high alkaline phosphatase and GGT calls for ultrasound of the bile ducts promptly, followed by MRI or endoscopic imaging if a blockage is suspected.