Fresh urine of alkaptonuric patient has a normally pale yellow colour but after prolonged contact with air or with alkaline substances (soap, bleach, ammonia, alkaline cleaning agents containing sodium and/or potassium hydroxide, etc.), it becomes dark grey or black. This is a typical sign for alkaptonuria and nearly always allows distinguishing it from other diseases associated with urine colour changes. For example, in another hereditary disease, congenital erythropoietic porphyrinuria (Morbus Günther) that also occurs early after the birth, urine has a typical red colouration. Similarly in haematuria or haemoglobinuria, urine has a pink to red colour. Urine sediment examination is decisive for making the diagnosis. In bilirubinuria, urine has a red-coffee colour (similar to ale) and dark coffee colour in melanuria. However, urine is always coloured in the fresh condition with the exception of alkaptonuria, in which it becomes darker in the open air after several hours or immediately after adding an alkaline substance.