Proteinuria is an abnormally high amount of protein in the urine.
The kidneys remove wastes from protein-rich blood through millions of tiny filtering screens called glomeruli. Most proteins are too large to pass through the glomeruli into the urine. The glomeruli are negatively charged, so they repel the negatively charged proteins. Thus, a size and charge barrier keeps protein molecules from entering the urine. But when the glomeruli are damaged, proteins of various sizes pass through them and are excreted in the urine.