Proteinuria (protein in the urine) is a sign of kidney disease. Many types of health problems can lead to kidney disease, which may be undetected until it becomes severe. Kidney disease is common in adults and can also occur in children, although the causes are usually different. Healthy kidneys do not allow protein to pass through their filtration systems, so if protein is found in the urine, it is a signal that further renal (kidney) testing is necessary, looking for an acute (short-term) kidney problem or for the presence of chronic kidney disease. Most persons with proteinuria have no symptoms. The presence of protein in the urine is found when a urinalysis (testing of a sample of urine) is done. Persons who have proteinuria usually see a nephrologist, a doctor with specialized education and training in treating diseases of the kidney. Urologists are doctors who specialize in surgical treatment of disorders of the kidneys and the urinary system. The February 3, 2010, issue of JAMA contains an article about proteinuria and using testing for proteinuria to improve diagnosis of chronic kidney disease.