Creatinine is a waste product from the normal breakdown of muscle tissue. As creatinine is produced, it's filtered through the kidneys and excreted in urine. Doctors measure the blood creatinine level as a test of kidney function. The kidneys' ability to handle creatinine is called the creatinine clearance rate, which helps to estimate the glomerular filtration rate (GFR) -- the rate of blood flow through the kidneys.
You experience any of these symptoms, which could be a warning sign of kidney disease. Unfortunately, some people have no symptoms of chronic kidney disease until the majority of kidney function is lost; that is why prevention -- through diet, exercise, and medication -- is key.