The kidneys filter blood through a mass of capillaries, called glomeruli, of which we have several million. The pores in the glomeruli are tiny so that water, sugar, sodium, other salts, and waste molecules can pass through them, but larger molecules, such as albumin, the major protein in blood, and blood cells cannot pass through. This filtrate then passes through a long tube leading from each glomerulus into the kidney's collecting system. More than that is abnormal and may indicate disease.
Many diseases can result in abnormal amounts of protein in the urine.
Probably the most common are diabetes and the various forms of glomerulonephritis, which damage the glomeruli so that excessive amounts of protein leak into the urine. It has been shown that the damage that diabetes inflicts on the kidneys can be controlled or minimized by tight control of blood sugar. Many doctors will routinely measure the amount of protein excreted in 24 hours by their diabetic patients to evaluate the damage that has occurred. Diabetes has become the most common cause of kidney failure leading to dialysis or kidney transplantation. Before the development of renal dialysis and renal transplant in the '60s, chronic renal failure was uniformly fatal.