Bilirubin in cats and dogs

What it shows

Most bilirubin — about 80% in eClinPath’s description — comes from the hemoglobin of worn-out red blood cells, the rest from other heme proteins such as myoglobin. Newly made bilirubin does not dissolve in water and travels bound to albumin: this is the indirect, or unconjugated, form. Liver cells attach it to glucuronic acid, creating the direct, or conjugated, form, which dissolves in water and leaves with bile.

Total bilirubin (TBIL) is the sum of all forms. Some forms also print direct bilirubin (DBIL), the conjugated part. In theory a larger direct share points to bile that cannot leave the liver, and a larger indirect share to excess production, as in hemolysis. In practice mixed patterns are common: Merck reports that in dogs the split did not reliably separate hemolytic from liver jaundice, and eClinPath considers it only somewhat useful, read together with the whole picture.

Higher than reference — what it may be a sign of

• Before the liver — too much pigment: destruction of red blood cells (hemolysis), for example immune-mediated hemolytic anaemia, Heinz body anaemia, zinc poisoning or blood parasites (Merck).
• In the liver: the liver cannot take up, process or release bilirubin — inflammation, fatty liver in cats, advanced liver disease.
• After the liver — bile cannot drain: a blocked bile duct or gallbladder outflow, or a rupture of the biliary tract.
• Other causes: sepsis; in a cat that has stopped eating, hepatic lipidosis can be behind a rising bilirubin (Merck).
• Sample artefacts: on some analyzers hemolysis and lipemia give a falsely high result (eClinPath).

Lower than reference

A low bilirubin on its own is not considered a sign of disease. The practical point is technical: bilirubin breaks down in light, so a sample stored in light for several days can read falsely low — which is why tubes are kept protected from light.

Look at together with

• Hematocrit and reticulocytes — a normal hematocrit makes hemolysis less likely, and a high reticulocyte count suggests it (VCA).
• ALP, GGT, ALT — liver and bile duct enzymes; raised together with bilirubin, they point to the liver or to bile flow.
• Urine bilirubin — only the direct form passes into urine. A rise in urine bilirubin can appear before blood values change (VCA).
• Bile acids — usually not measured when bilirubin is already high without hemolysis: bile acids would be high anyway and add nothing (Merck).

Units and conversion

European and Russian labs report µmol/L (мкмоль/л), North American labs mg/dL (мг/дл). One mg/dL equals 17.1 µmol/L, so 1.0 mg/dL is about 17 µmol/L. Direct bilirubin uses the same units and the same factor. In the app each row keeps the unit printed on the form; the chart converts to one unit so forms from different labs line up.

Reference range

The reference range is on your lab’s form — for total bilirubin and, if printed, for direct bilirubin separately. Visible jaundice usually appears once total bilirubin exceeds about 2.5–3 mg/dL, roughly 43–51 µmol/L (Merck, eClinPath), so a value above the range does not always show as yellow gums. After a bile flow problem is resolved, jaundice can last 7–14 days, because part of the pigment is bound to albumin and leaves the blood slowly (Merck).

Cats and dogs

Sources

1. eClinPath (Cornell University) — Bilirubin
2. Miscellaneous Serum Biochemical Measures in Hepatic Disease in Small Animals. Merck Veterinary Manual
3. Clinical Biochemistry. Merck Veterinary Manual
4. VCA Animal Hospitals — Testing for Liver Disease
5. Hepatic Function Tests in Small Animals. Merck Veterinary Manual