Homogentisic acid

Homogentisic acid (HGA) is an intermediate in the breakdown (catabolism) of the amino acids tyrosine and phenylalanine. In healthy metabolism, the enzyme homogentisate 1,2-dioxygenase (HGD) converts HGA to the next step in the pathway, so only trace or undetectable amounts appear in urine. Labs measure HGA in a spot urine sample and report it normalized to creatinine (mmol HGA per mol creatinine) to account for urine concentration.

Why it matters

• Markedly elevated HGA is a hallmark of alkaptonuria, a rare inherited (autosomal recessive) condition caused by variants in the HGD gene. When HGD activity is low or absent, HGA builds up and is excreted in large amounts. Over years, oxidized HGA can deposit in tissues (ochronosis), contributing to early spine and large-joint osteoarthritis, tendon/ligament issues, heart-valve calcification, and a higher risk of kidney or prostate stones.

• Normal/undetectable HGA is expected in most people and supports intact tyrosine metabolism.

How to read your result (always use your lab’s reference range)

• Normal / Undetectable: Typical finding; no evidence of excess HGA. No specific action is needed unless symptoms or other organic acids suggest a metabolic issue.

• Elevated:

  • Borderline/near-cutoff: Consider repeat testing on a well-collected first-morning urine to confirm and review pre-analytic factors (hydration, collection, transport).

  • Clearly/markedly high: Strongly consider alkaptonuria. Your clinician may order targeted quantitative HGA testing, evaluate for clinical signs (e.g., urine darkening on standing, early joint disease), and discuss HGD genetic testing. Diagnosis is not based on one value alone; confirmation is standard.

Symptoms and clinical clues that prompt follow-up

• Infants/children: Diapers or urine that darken on standing or with alkali; later, joint stiffness.

• Adults: Early-onset back or large-joint pain, reduced spine flexibility, tendon/ligament problems, bluish-black cartilage discoloration (ears), and history of renal/prostate stones.

What can influence the result

• Hydration/collection: The creatinine ratio helps, but extremely dilute or concentrated urine can still shift values. Follow collection and shipping instructions closely.

• Diet/supplements: Typical dietary tyrosine/phenylalanine does not raise HGA above reference range when HGD function is normal.

• Medications/therapies: Nitisinone (used selectively in alkaptonuria) dramatically lowers urinary HGA; it can raise blood tyrosine, so it requires medical supervision and dietary oversight. Always tell the lab and your clinician about current therapies.

What to do next (actionable steps)

• Normal/undetectable HGA: No HGA-specific action needed. Interpret alongside symptoms and the rest of the Organic Acids panel.

• Borderline elevation:

  • Repeat the test (first-morning urine, proper handling).

  • Review collection, medications, supplements.

  • Discuss whether quantitative HGA and/or genetic counseling/testing is appropriate based on history and family background.

• Clearly elevated:

  • Consult a clinician experienced in inherited metabolic conditions.

  • Consider confirmatory urine/plasma HGA and HGD genetic testing.

  • Management is individualized; it may include specialist follow-up for joints/spine/heart, physical therapy and pain strategies, and—in appropriate cases—nitisinone under specialist care.

  • Support joint health with healthy body weight, muscle strengthening, and low-impact exercise as tolerated.

Key takeaways

• What it is: A urine marker of the tyrosine/phenylalanine pathway, reported as mmol/mol creatinine.

• Normal result: Typical in most people.

• High result: Think alkaptonuria—confirm with repeat/quantitative testing and consider HGD genetics.

• Carrier detection: Routine urine HGA is not a dependable carrier screen.

• Vitamin C: Not proven harmful in alkaptonuria; potential antioxidant role is uncertain—use only within a clinician-guided plan.

FAQs

Does one high result diagnose alkaptonuria?
No. It’s a strong clue, but clinicians confirm with repeat/quantitative HGA and usually HGD genetic testing.

Can food alone cause high HGA?
Not in people with normal HGD enzyme function. Marked elevations generally reflect enzyme deficiency, not diet.

My urine sometimes looks dark—do I have alkaptonuria?
Many factors can darken urine. The classic sign is urine that darkens on standing, plus a high HGA result. Testing clarifies.

Should my family get tested?
Because alkaptonuria is autosomal recessive, consider genetic counseling if someone in the family is diagnosed.