How is it that protein in the urine may be a sign of kidney damage?

Proteinuria: To better answer your question, I have to know the amount of proteinuria (p). If it were discovered in a urine analysis by a dip stick, that does not quantify it. A 24 hour urine is done to see how much p you have in a day. If you have.

Altered filtration.: The kidney normal has a complex filtering & absorption system that retains almost all of the protein in the blood to conserve it. When the cells that form the filter develop problems through inflammation or disease, they allow protein to the released in the urine detectable on urinalysis. An evaluation to assess the degree and to find the cause is important to limit any kidney damage.

Nephron damage: Think of the kidney as a filter. This filter is intended to wash out the impurities in your blood, but designed to retain the proper nutrients, blood cells, proteins, etc your body requires. When there is damage to these filtration units (nephrons) in the kidneys, the filtration becomes less efficient and proteins are commonly lost into the urine.