Bilirubin crystals occur with increased concentrations of conjugated bilirubin in the urine, which can occur with severe liver disease, cholestasis or increased hemolysis. These crystals usually appear as small bundles of fine, needle-shaped structures, but may take other irregular shapes, such as round spheres, granules, cubes or rhombic plates. The color ranges from yellow to reddish-brown. They are usually seen in acidic urine with negative or very weak birefringence. Bilirubin crystals can be either loose or incorporated into leukocytes, epithelial cells or cylinders. Because of their color and shape, they can be confused with other crystals such as ammonium biurate, but these occur only in alkaline urine, which helps distinguish them.