Diagnostic Testing for Alkaptonuria
The definitive test for alkaptonuria is a urine organic acid analysis by gas chromatography-mass spectrometry (GC-MS) to detect elevated homogentisic acid levels. 1
Initial Diagnostic Approach
Clinical Suspicion
- Look for key clinical manifestations:
- Dark urine that turns black upon standing (oxidation of homogentisic acid)
- Blue-dark pigmentation of connective tissue (ochronosis) - typically visible around age 30 in ears and eyes
- Joint pain and arthropathy - typically develops around age 50
- History of tendon/ligament ruptures
- Cardiovascular or renal complications
Laboratory Testing Algorithm
First-line test: Urine Organic Acid Analysis
- Collect a random urine sample (early morning preferred)
- Send for comprehensive organic acid analysis by GC-MS
- This will detect elevated homogentisic acid, which is pathognomonic for alkaptonuria 1
Confirmatory testing options:
- Repeat urine organic acid analysis if initial results are equivocal
- Genetic testing for mutations in the HGD gene (homogentisate 1,2-dioxygenase)
Simple screening test:
- Add sodium hydroxide to urine sample - turns dark brown/black in alkaptonuria
- This can be used as a quick screening but requires confirmation with GC-MS
Technical Considerations
Urine Collection and Processing
- Early morning urine sample is preferred
- Samples should be processed promptly or refrigerated to prevent degradation
- Quantitative analysis should be performed in an accredited laboratory 1
- Results should be expressed as a ratio to creatinine for random samples
Genetic Testing
- Molecular analysis of the HGD gene can identify causative mutations
- Most common mutations include c.175delA, c.674G>A, and c.1007-2A>T 2
- Genetic testing is particularly useful for:
- Confirming diagnosis in equivocal cases
- Family screening
- Genetic counseling
Differential Diagnosis
When evaluating dark urine, consider:
- Medications causing urine discoloration
- Porphyrias
- Melanuria
- Hemoglobinuria
- Myoglobinuria
Clinical Pearls and Pitfalls
- Key Pearl: Alkaptonuria is often asymptomatic in childhood with only dark urine as the presenting sign 3
- Common Pitfall: Delayed diagnosis due to failure to investigate dark urine in childhood
- Important Consideration: Symptoms may develop in multiple organ systems beyond the classic triad, including neurological findings and depression 2
- Testing Recommendation: Always confirm positive screening tests with quantitative laboratory analysis 1
Disease Monitoring
Once diagnosed, monitor:
- Urinary homogentisic acid levels to assess disease burden
- Joint function and arthropathy progression
- Cardiovascular status
- Renal function
Special Populations
- Children: May only present with dark urine; early diagnosis allows for earlier intervention
- Adults: May present with joint pain, tendon ruptures, or cardiovascular complications
- Pregnant women: No specific testing modifications needed
By following this systematic approach to testing for alkaptonuria, clinicians can establish an early diagnosis, potentially allowing for earlier intervention with treatments like nitisinone that can reduce homogentisic acid production and potentially slow disease progression.