Protein in the urine (proteinuria) is seen in many forms of chronic kidney disease (CKD), but it usually suggests damage to the filters of the kidney (known as glomeruli). Kidney diseases that cause damage to the glomeruli are known as glomerulonephritis. Damage can occur in other areas, such as the tubules and inner parts of the kidneys known as the medullary interstitium. These kidney diseases are known as interstitial kidney disease. If damage occurs in these latter areas, proteinuria may not be seen as kidney damage progresses. It is sometimes necessary to perform a kidney biopsy in order to know what areas of the kidney are being damaged in order to know how best to treat the patient with CKD. Proteinuria is a common marker that is followed over time to assess the degree of damage that is occurring. That is why we recommend that both the estimated glomerular filtration rate (eGFR) be measured and followed as well as the urine testing to look for the magnitude of proteinuria.