Alkaptonuria is a rare inherited disorder known as black urine disease, wherein a person's urine turns dark brown or black when exposed to air. This condition is a fallout of the body's preventing the complete breakdown of two amino acids, tyrosine and phenylalanine, which are involved in building protein. The body cannot produce an enzyme called homogentisic dioxygenase acid (HGD). The bones and cartilage become discoloured and brittle, leading to the onset of another condition called osteoarthritis, especially in the spine and large joints. Hence, alkaptonuria can be defined as a rare genetic inborn error of protein metabolism. In the following section, let’s learn about the alkaptonuria diagnostic test.
What are the Diagnoses for Alkaptonuria?
The doctor may conduct tests to determine alkaptonuria if a patient shows symptoms such as the early onset of osteoarthritis, or their urine is turning black when exposed to air. The patient’s family history also comes into the picture. The tests include:
- Gas chromatography - Mass spectroscopy (GC-MS) to look for elevated levels of homogentisic acid in a patient’s urine within a 24-hour period. The normal content per day should be 1-8 grams
- Echocardiography to detect cardiac-related problems
- Molecular genetic testing to identify any mutated HGD gene
For a better diagnosis of alkaptonuria, the doctor may want to know more about a patient’s family history, which is very useful in correctly diagnosing alkaptonuria. Most people do not know whether they carry the HGD gene or their parents were the inadvertent carriers.
Biochemical test: This is the gold standard method to check for alkaptonuria. A first-morning urine sample taken from the patient is mixed with ferric chloride. The doctor ascertains the condition as alkaptonuria if the mixture turns black.