What causes low levels of bilirubin in the blood?

Patients who are experiencing abnormally low levels of bilirubin in their blood may have been affected adversely by medications such as phenobarbital, theophylline or vitamin C, according to WebMD. Caffeine also interferes with the accuracy of a bilirubin test, as caffeine is known to lower bilirubin levels as well.

Bilirubin is a brownish yellow substance produced when the liver breaks down red blood cells, notes WebMD. Conjugated bilirubin is the water soluble form of the substance, created in the liver from its unconjugated state. A doctor performs a bilirubin test through a blood sample to measure the amount of bilirubin present within a patient's body.