The first symptom most AKU patients will experience is black or dark urine. Large quantities of a chemical called homogentisic acid (HGA) are excreted in the urine. If left to stand and exposed to the air, the HGA in urine will oxidise and start to turn black. The length of time needed for the urine to turn black can vary from patient to patient but nearly all patients’ urine will eventually turn black.
Black urine does not cause any physical harm. However, it can be used as a diagnostic tool. Babies born with AKU may have dark or red staining in their nappy as a result of HGA in the urine. Doctors should then carry out a urine or blood test to check the level of HGA in the baby’s body in order to confirm a diagnosis of AKU1
Published: 27.02.23 Next review: 27.02.26