Pregnancy SmartSite^TM

• Home
• Email
• Site Map
• Contact

• Normal Pregnancy
• Labor & Delivery
• Complications
• After Your Child Is Born

Normal Pregnancy

Staying Healthy

Alcohol and pregnancy Managing your weight gain in pregnancy Steps to take before you get pregnant When you need to gain more weight during pregnancy

What to expect during pregnancy

Aches and pains during pregnancy Choosing the right practitioner Common symptoms during pregnancy Hyperemesis Gravidarum Morning sickness Pregnancy and travel Pregnancy and work Problems sleeping during pregnancy Skin and hair changes during pregnancy Teenage pregnancy

Preparing to go home with your baby

Cribs and crib safety Eat right during pregnancy Preparing your other children

Visits and Tests

Amniocentesis Chorionic villus sampling Genetic counseling before pregnancy Glucose tolerance test-pregnancy Monitoring your baby before labor Nuchal translucency Prenatal care in your first trimester Prenatal care in your second trimester Prenatal care in your third trimester

Alkaptonuria

Definition

Alkaptonuria is a rare condition in which a person's urine turns a dark brownish-black color when exposed to air. Alkaptonuria is part of a group of conditions known as an inborn error of metabolism.

Causes

A change in the HGD gene causes alkaptonuria.

The change in the gene makes the body unable to properly break down the 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 through families. If both parents carry a nonworking copy of the gene related to this condition, each of their children has a 25% (1 in 4) chance of developing the disease. This is called autosomal recessive inheritance.

Symptoms

Urine in an infant's diaper may darken and can turn almost black after several hours. However, many people with this condition may not know they have it. The disease is most often discovered in 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 and cornea

Exams and Tests

A urine test is done to test for alkaptonuria. If ferric chloride is added to the urine, it will turn the urine black in people with this condition.

Treatment

Management of alkaptonuria has traditionally been focused on controlling symptoms. Eating a low protein diet may be helpful, but many people find this restriction difficult. Medicines, such as NSAIDs and physical therapy may help relieve joint pain.

Clinical trials are underway for other medicines to treat this condition and to assess whether the drug nitisinone provides long-term help with this illness.

Outlook (Prognosis)

The outcome is expected to be good.

Possible Complications

The build-up of homogentisic acid in the cartilage causes arthritis in many 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 life in people with alkaptonuria.

• Kidney stones and prostate stones may be more common in people with alkaptonuria.

When to Contact a Medical Professional

Contact 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.

Prevention

Genetic counseling is recommended for people with a family history of alkaptonuria who are considering having children.

A blood test can be done to see if you carry a non-working copy of the gene that causes alkaptonuria.

Prenatal tests (amniocentesis or chorionic villus sampling) can be done to screen a developing baby for this condition if the genetic change has been identified.

References

Dietzen DJ, Willrich MAV. Amino acids, peptides, and proteins. In: Rifai N, Chiu RWK, Young I, Burnham Carey-Ann D, Wittwer CT, eds. Tietz Textbook of Laboratory Medicine. 7th ed. St Louis, MO: Elsevier; 2023:chap 31.

Kliegman RM, St. Geme JW, Blum NJ, et al. Defects in metabolism of amino acids. In: Kliegman RM, St. Geme JW, Blum NJ, et al, eds. Nelson Textbook of Pediatrics. 22nd ed. Philadelphia, PA: Elsevier; 2025:chap 105.

Merritt JL, Gallagher RC. Inborn errors of carbohydrate, ammonia, amino acid, and organic acid metabolism. In: Gleason CA, Sawyer T, eds. Avery's Diseases of the Newborn. 11th ed. Philadelphia, PA: Elsevier; 2024:chap 29.

National Institutes of Health, National Library of Medicine. Long-term study of nitisinone to treat alkaptonuria. classic.clinicaltrials.gov/ct2/show/NCT00107783. Updated August 26, 2021. Accessed April 15, 2025.

Riley RS, McPherson RA. Basic examination of urine. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Elsevier; 2022:chap 29.

Review Date: 4/8/2025

Reviewed By: Anna C. Edens Hurst, MD, MS, Associate Professor in Medical Genetics, The University of Alabama at Birmingham, Birmingham, AL. Review provided by VeriMed Healthcare Network. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.