urea and creatinine retained in the blood in nephropathy

[jnkdna]

I wasn't sure where exactly to post this question. I just wanted to know, when a patient is suffering from diabetes and gets nephropathy, it is due to poor filtration in the glomerulus. therefore proteins like microalbumin appear in the urine. however, urea and creatinine are retained in the blood and their levels increase. My question is, why doesnt the levels of urea and creatinine increase in the urine due to poor filtration.