What Is Jaundice?

Jaundice, also called icterus, is the yellow discoloration of the skin, sclerae, and mucous membranes caused by an accumulation of bilirubin in blood and tissue. It is a clinical sign rather than a disease in itself, and it indicates that the production, conjugation, or excretion of bilirubin has been disrupted somewhere along the pathway that clears the pigment from the body. In adults, visible yellowing generally appears once total serum bilirubin exceeds roughly 2.5 to 3 mg/dL, about twice the upper limit of the normal range.

Bilirubin is the end product of heme catabolism. Senescent red blood cells are broken down in the reticuloendothelial system, releasing heme that is converted to unconjugated bilirubin, a lipophilic molecule that travels bound to albumin. The liver takes it up, conjugates it with glucuronic acid to make it water-soluble, and excretes it into bile. Any failure of that sequence raises circulating bilirubin, and the point of failure determines the clinical classification.

Classification and Underlying Causes

Clinicians divide jaundice into three categories according to where the disruption occurs. Prehepatic jaundice follows excessive red cell destruction, as in hemolytic anemia, sickle cell disease, or transfusion reaction, and produces mainly unconjugated bilirubin. Hepatic jaundice arises from injury or inherited enzyme deficiency within the liver itself, including viral hepatitis, cirrhosis, drug toxicity, and Gilbert syndrome. Posthepatic or obstructive jaundice results from blockage of the biliary tree by gallstones, strictures, or pancreatic tumors, and raises conjugated bilirubin along with alkaline phosphatase. Neonatal jaundice is a distinct and far more common case: as reviewed in the NCBI Bookshelf chapter on neonatal jaundice, roughly 60 percent of term and 80 percent of preterm infants become visibly jaundiced within the first week of life because hepatic conjugating capacity is still immature.

Measurement and Screening

Diagnosis rests on quantifying bilirubin rather than on visual inspection, which is unreliable across skin tones and lighting conditions. The reference method is a serum assay performed on a heel-stick or venous sample, reported as total and direct fractions. Because repeated blood sampling in newborns is painful and slow, transcutaneous bilirubinometry has become the standard screening tool. The instrument illuminates the skin, collects diffusely reflected light at several wavelengths, and separates the optical signature of bilirubin from those of hemoglobin and melanin. Refinements to that inverse problem, such as photon diffusion models of light transport in neonatal skin, improve agreement with serum values in darker-skinned infants, where earlier empirical calibrations tended to overestimate. Smartphone cameras with controlled illumination are being evaluated as a lower-cost variant of the same optical measurement.

Phototherapy and Management

Treatment of adult jaundice targets the underlying disorder, whether that means antiviral therapy, biliary stenting, or surgical removal of an obstruction. Neonatal hyperbilirubinemia instead has a direct physical therapy. Blue light in the 460 to 490 nm band converts unconjugated bilirubin in the skin into water-soluble photoisomers, chiefly lumirubin, which the infant can excrete without hepatic conjugation. A review of neonatal phototherapy describes how irradiance, spectral output, and exposed body surface area govern the rate of bilirubin decline, and light-emitting diode arrays have largely replaced fluorescent tubes because they deliver narrowband output with less heat. Exchange transfusion remains the fallback when bilirubin approaches levels associated with kernicterus and permanent neurological injury.

Applications

Jaundice is a focus of work in a range of technical fields, including:

• Optical biomedical instrumentation and transcutaneous sensing
• Point-of-care and smartphone-based diagnostic devices
• LED light source design for phototherapy systems
• Clinical laboratory assay development and standardization
• Machine learning applied to image-based neonatal screening
• Global health programs in settings without laboratory infrastructure