Alkaptonuria is a rare condition in which a person's urine turns a dark brownish-black color when exposed to air.

See also: Inborn error of metabolism

A defect in the HGD gene causes alkaptonuria.

The gene defect makes the body unable to properly break down certain amino acids (tyrosine and phenylalanine). As a result, a substance called homogentisic acid builds up in the skin and other body tissues. The acid leaves the body through the urine. The urine turns brownish-black when it mixes with air.

Alkaptonuria is inherited, which means it is passed down from parents to their children. To get this disease, each of your parents must pass you a copy of the faulty HGD gene.

Urine in an infant's diaper may darken and can turn almost black after several hours. However, many persons with this condition may not know they have it until mid-adulthood (around age 40), when joint and other problems occur.

Symptoms may include:

• Arthritis (especially of the spine) that gets worse over time
• Darkening of the ear
• Dark spots on the white of the eye (sclera) and cornea

A urine test (urinalysis) is done to test for alkaptonuria. If ferric chloride is added to the urine, it will turn the urine a black color in patients with this condition.

Some patients benefit from high-dose vitamin C. This has been shown to decrease the build-up of brown pigment in the cartilage and may slow the development of arthritis.

The outcome is expected to be good.

People with this condition also can get arthritis in adulthood. The build-up of homogentisic acid in the cartilage causes arthritis in about 50% of older adults with alkaptonuria.

• Homogentisic acid also can build up on the heart valves, especially the mitral valve. This can sometimes lead to the need for valve replacement.
• Coronary artery disease may develop earlier in people with alkaptonuria.
• Kidney stones and prostate stones may be more common in people with alkaptonuria.

Call your health care provider if you notice that your own urine or your child's urine becomes dark brown or black when it is exposed to air.

There is no know prevention.

Chakrapani A, Holme E. Disorders of tyrosine metabolism. In: Fernandes J, Saudubray J-m, van den Berghe G, Walter JH, eds. Inborn Metabolic Diseases: Diagnosis and Treatment. 4th ed. New York, NY: Springer;2006:chap 18.