How is alkaptonuria diagnosed?
The clinical diagnosis is suggested by the following typical triad of findings:
• Degenerative arthritis (premature), especially in the spine with disc calcification; the earliest involvement is observed in the lumbar spine
• Abnormal pigmentation, usually occurring first in the ear cartilage and sclerae
• Urine that turns blue-black on standing (occurs in 75% of patients).
HA binds collagen and is, therefore, deposited in connective tissues throughout the body. Observation of blue-black or gray-brown pigment in the tissues (skin, ears, cerumen, sclerae, cartilage) suggests ochronosis. The diagnosis can be further supported if the urine darkens on standing in air or on addition of alkali (10 drops of 10% NaOH to 20 mL of urine). However, confirmation of the diagnosis requires measurement of 24-hour urinary excretion of HA. A specific enzyme assay for HGD can also be performed.
