If the colour of the urine is red, the presence of blood cells among the urine is usually considered. When the integrity of the filtration membrane of the glomerulus is reduced and permeability is increased due to inflammation, injury, destruction of immune complexes, and damage to the renal tubules, this can cause excessive red blood cells to enter the urine, giving it a washed-out or blood-coloured appearance, and the presence of associated underlying diseases such as glomerulonephritis and IgA nephropathy needs to be considered.
On the other hand, when there is damage to the ureter or urethra, stones or tumours, this can also lead to blood flowing out with the urine, resulting in a reddish colour.