The stages of chronic kidney disease are determined by the glomerular filtration rate. Glomerular filtration is the process by which the kidneys filter the blood, removing excess wastes and fluids. Glomerular filtration rate (GFR) is a calculation that determines how well the blood is filtered by the kidneys. It is one way to measure kidney function.
Glomerular filtration rate is usually calculated using a formula that includes a person's age, gender, race, and serum creatinine levels. A GFR under 60 mL/min/1.73 m² may indicate kidney disease. The lower the GFR number, the worse the kidney function. This number is an estimate. It may not be a good measure of kidney health in some people, such as the very young or very old, amputees, or obese people. In some cases, GFR may also be estimated with a 24-hour urine collection.
Chronic kidney disease is defined as either kidney damage or GFR of less than 60 for longer than 3 months.