Patients have high bilirubin levels for many reasons, almost always indicative of issues with the liver. Newborns often have high bilirubin levels because their liver takes a few days after birth to "wake up" and do what a liver is supposed to do, mainly cleanse the blood as it comes from the digestive tract. For adults, it's often indicative of liver disease, tumors, cancer, sclerosis, hemolytic anemia, blockage of the bile ducts and other liver pathologies.

Bilirubin is a pigment and a by-product of digestion and red blood cell destruction. When the liver isn't working properly, the blood isn't being filtered of waste products. Because it is a pigment, when significantly elevated, patients appear jaundiced, i.e., their skin and whites of their eyes take on an amber color. Normal bilirubin levels are well below 2.0 mg/dl. Concentrations above 15 mg/dl lead to brain damage in neonates.

Since bilirubin degrades when exposed to light by as much as 50 percent per hour, it's critical to protect blood samples from exposure to daylight and artificial light as soon as the sample is collected. Since excessive bilirubin concentrations can cause brain damage, failure to protect samples from light can prevent infants from receiving critical phototherapy (bili-lights) that break down bilirubin at the capillary level.

Bilirubin exists in two forms: conjugated and unconjugated. Conjugated bilirubin is measured by the direct bilirubin test, while unconjugated bilirubin is measured by an indirect bilirubin assay. When elevated, each form is diagnostic of different diseases and conditions, and can assist physicians in pinpointing the cause of an elevated total bilirubin result.

Samples for bilirubin are typically serum samples, and have a stability of about two days at room temperature. Prolonged contact with red blood cells does not seem to affect test results. Devices exist that measure bilirubin transdermally without the need for a blood sample.