Low bilirubin: what a low total bilirubin means and whether to worry

If your blood test shows total bilirubin flagged low, the most likely explanation is ordinary: your body breaks down a little less heme (the iron-carrying part of red blood cells), clears bilirubin a little faster, or your lab draws its lower line higher than most. Bilirubin is the yellow pigment left over when old red blood cells are recycled, and the liver problems doctors look for on a liver panel all push it up, not down. This page explains what low bilirubin generally means; your own result is best read by your clinician alongside the rest of your tests and history. For the full marker, see total bilirubin.

What counts as low bilirubin levels, in mg/dL and µmol/L

MedlinePlus, the US National Library of Medicine's consumer guide, gives a typical adult total bilirubin range of about 0.1 to 1.2 mg/dL ¹. Each lab sets its own range from the healthy people it tests, so the lower limit on a report can be 0.1, 0.2 or 0.3 mg/dL. That is why a total bilirubin of 0.2 mg/dL is flagged at one lab and passes without comment at another. Outside the US, bilirubin is reported in µmol/L: multiply mg/dL by 17.1 to convert ².

Why a lower number happens: where bilirubin comes from

An adult makes roughly 250 to 400 mg of bilirubin a day, and about 80% of it comes from the heme in worn-out red blood cells ². The amount in your blood at any moment reflects two rates: how fast bilirubin is made and how fast the liver clears it. Less production or faster clearance gives a lower number.

Why is my bilirubin low? The common reasons

Most low results have one of the explanations below. Each comes with the clue that usually points to it.

1. 1Your lab's rangeA lower limit of 0.3 mg/dL flags results that another lab would call normal. How to tell: compare the reference range printed on your report with an older report from a different lab.
2. 2Being femaleWomen run lower bilirubin than men. In a UK cohort of about 500,000 adults, the median was 0.53 mg/dL in women and 0.64 mg/dL in men ³. How to tell: a low-normal result in a woman with an otherwise normal panel needs no other explanation.
3. 3SmokingSmokers have lower bilirubin than non-smokers. In 715 middle-aged men having a coronary angiogram (an X-ray of the heart arteries), 42% of current smokers had bilirubin in the lowest quarter of values, against 16% of people who had never smoked ⁶. How to tell: you smoke now.
4. 4Medicines that speed up bilirubin clearancePhenobarbital and some other anti-seizure drugs can switch on UGT1A1, the liver enzyme that clears bilirubin, so it leaves the blood faster. How to tell: you take one of these medicines. The low bilirubin is an expected effect; do not stop or change the medicine without your prescriber.
5. 5Fewer red cells being broken downIron-deficiency anemia means fewer and smaller red cells, which can mean less heme reaching the liver and a lower bilirubin. How to tell: hemoglobin or ferritin is low on the same panel, and you may feel tired or short of breath.
6. 6A sample left in the light or processed lateBilirubin breaks down in light, so a tube that sat uncovered or waited hours to be spun can read lower than it should. How to tell: a one-off low result that is normal on a repeat.
7. 7You don't have Gilbert syndromeA minority of people have Gilbert syndrome, a harmless inherited trait that slows UGT1A1 and keeps bilirubin on the high side. Lab ranges include these people, so if you don't have it, your bilirubin naturally sits lower in the range. How to tell: you have never had a high bilirubin result.

For how the blood count and iron tests fit together, see anemia, iron deficiency and the iron and anemia topic.

Is low bilirubin bad? What the heart and lung research shows

Bilirubin is an antioxidant: at the levels found in blood it mops up the reactive molecules that damage fats and cell walls ⁹. That made researchers ask whether people with less of it do worse. In the largest study, Horsfall and colleagues followed 504,206 UK primary-care patients with normal-range bilirubin. Each 0.1 mg/dL higher bilirubin went with an 8% lower rate of lung cancer in men (11% in women), a 6% lower rate of COPD and a 3% lower death rate from any cause ³.

Among patients with normal-range bilirubin levels in primary care practices, relatively higher levels of bilirubin were associated with a lower risk of respiratory disease and all-cause mortality.Horsfall LJ et al., JAMA 2011 ³

Heart and brain studies point the same way. In a US survey of 13,214 adults, each 0.1 mg/dL higher bilirubin went with 9% lower odds of having had a stroke ⁸. In a small Czech study of 50 people over 40 with Gilbert syndrome, whose bilirubin runs high, 2% had ischemic heart disease, against 12.1% of the general population ⁷.

An observational link cannot say which way the arrow points. Smoking, inflammation and poor metabolic health all lower bilirubin and all raise heart risk on their own. To separate the two, Stender and colleagues used Mendelian randomization, which compares people who inherited a UGT1A1 variant that sets their bilirubin higher from birth. Among 67,068 Danish adults, the variant nearly doubled bilirubin (95% higher) yet left the odds of ischemic heart disease unchanged (odds ratio 1.03, 95% CI 0.96 to 1.11) ⁴. If bilirubin itself protected the heart, those people should have had less heart disease, and they did not. The genetic test covers heart disease only; for lung disease and death the question is still open.

If your bilirubin is low, the heart markers you can change, such as HDL and hs-CRP, are more useful to look at. Raising the bilirubin number itself has no proven benefit. The heart and cholesterol topic shows how to read these together.

Does low bilirubin cause symptoms?

Low bilirubin causes no symptoms. If you feel tired, short of breath or pale, the cause is somewhere else, most often anemia, and the hemoglobin and ferritin results are the place to look. Symptoms tied to bilirubin come from a high level: yellow skin or eyes, dark urine and pale stools.

What to check alongside a low result

The American College of Gastroenterology's guideline on abnormal liver tests works up raised bilirubin; it does not list low bilirubin as a finding ⁵. When the rest of the liver panel and the blood count are normal, a low total bilirubin is very unlikely to signal a problem. Direct (conjugated) bilirubin is usually tiny, so a low or normal value there fits a low total. The liver enzymes guide explains how the panel reads together.

Can you raise low bilirubin, and should you?

No supplement or diet has been shown to raise total bilirubin safely, and the genetic evidence gives no reason to expect a heart benefit if it did ⁴. Fasting can raise bilirubin for a day or two, which is why a fasting draw may read a little higher, but the effect is short-lived and has no health benefit. Smoking goes with lower bilirubin, and in the same study former smokers looked more like people who had never smoked ⁶; stopping smoking lowers heart and lung risk whatever it does to the number. If a medicine that speeds up bilirubin clearance is the reason, the low bilirubin on its own is not usually a reason to change treatment; any decision about the medicine belongs with your prescriber.

If you searched for how to lower bilirubin because your result was high, the causes and next steps are on the main total bilirubin page.

When a low bilirubin is worth raising with a doctor

Otherwise, see a doctor within days or weeks in these cases:

• Low bilirubin together with low hemoglobin or low ferritin, or with tiredness or breathlessness you cannot explain.
• Low bilirubin together with abnormal liver enzymes: ALT, AST (aspartate aminotransferase), GGT or ALP (alkaline phosphatase). In that case the liver enzymes are the finding to discuss.
• A large drop from your own earlier results at the same lab, with no new medicine and no change in smoking.

Outside those cases, a low total bilirubin usually needs no follow-up; your clinician, who sees your full results and history, can confirm that for you. If you want to recheck a one-off low reading, wait a few months, use the same lab, draw it at the same time of day in the same fasting state, and ask that the tube be kept out of the light. The blood test prep guide covers the rest, and more on the organ itself lives in the liver topic.

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