Homogentisic acid oxidase deficiency

Alternative names 
Alcaptonuria; Alkaptonuria; Ochronosis

Alkaptonuria is a rare inherited disorder of metabolism characterized by urine which turns black when exposed to air. Another characteristic is the development of arthritis in adulthood.

Causes, incidence, and risk factors

Alkaptonuria is an autosomal recessive inherited disorder. In affected individuals, an amino acid known as tyrosine is not properly metabolized, due to a defect in an enzyme called homogentisic acid oxidase.

Because of the defect, homogentisic acid is excreted in the urine and turns a brown color upon exposure to air. This is the result of a dark pigment with an ochre color (earthy red or yellow), which led to the name ochronosis. The bones and cartilage of the body can be brown colored.


  • Family history of alkaptonuria  
  • Urine in diaper may darken after several hours (can become almost black)


  • Progressive arthritis, especially of the spine  
  • Darkening of the ear (from black staining of the cartilage in the ear)  
  • Dark spots on the sclera (white of the eye) and cornea

Signs and tests
Urinalysis is positive for reducing substance. Further urine testing shows a positive ferric chloride test.


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

Expectations (prognosis)
The outcome is expected to be good.


  • Accumulation of homogentisic acid products in the cartilage causes arthritis in about 50% of older adults with alkaptonuria.  
  • Homogentisic acid products can accumulate on the heart valves, especially the mitral valve, sometimes leading to the need for valve replacement.  
  • Coronary artery disease may develop earlier in people with alkaptonuria.  
  • Kidney and prostate stones may be more common in people with alkaptonuria.

Calling your health care provider
Call your health care provider if you notice that your own urine or your child’s urine becomes dark brown or black upon exposure to air.

Like most genetic diseases, alkaptonuria itself cannot be prevented, but some of the manifestations, such as arthritis, can be minimized by treatment.

Johns Hopkins patient information

Last revised: December 5, 2012
by Potos A. Aagen, M.D.

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