What is the Glomerular Filtration Rate?

Glomerular filtration rate (GFR) is a test used to measure the functions of the kidneys, specifically how well structures call glomeruli are filtering blood. The glomeruli are vital units inside the kidneys responsible for removing waste substances from the blood. The glomerular filtration rate test is often performed to help diagnose kidney disease, can can indicate the stage the disease is in. It is also an important test to evaluate the progression of kidney disease in many patients.

Calculation of glomerular filtration rate is often done based on the profile of the patient and the level of creatinine in the blood. A patient's profile includes age, gender, and weight, among other factors. Men naturally have higher creatinine levels in the blood than women, so gender is an important part of the GFR calculation. Age is also a factor, because as an individual reaches the age of around 30, the glomerular filtration rate starts to decrease.

Creatinine is a waste product from muscle activities, and is usually cleared by the kidneys for elimination through the urine. When the glomeruli are not functioning well, creatinine can build up in the blood in large amounts. Symptoms of high creatinine in the blood include fatigue, confusion, and frequent urination, among many other nonspecific symptoms. Determination of creatinine levels is done in the laboratory after extraction of an adequate sample of blood in the arm.

GFR is reported in milliliters per minute per 1.73 m², which is the average surface area of an adult. The normal value for glomerular filtration rate is about 120 to 130 ml/min/1.73 m². Values falling below the normal rate frequently indicate the presence of kidney problems. A value of 90 ml/min/1.73 m² or above can mean an increase risk for the development of kidney problems. Risk factors include hypertension or high blood pressure, diabetes, and history of kidney diseases in the family.

When the GFR levels are persistently below 60 ml/min/1.73 m², it could be an indication of chronic kidney disease. Chronic kidney disease symptoms include swelling of the face, hands, legs, and abdomen, and can progress to kidney failure if not managed. Levels below 15 ml/min/1.73 m² are often dangerous, as it signals kidney failure, which requires dialysis and kidney transplant for treatment.

Aside from glomerular filtration rate, kidney function can also be evaluated using the creatinine clearance test. The creatinine clearance test measures the creatinine levels in the blood and a 24-hour urine sample. The process is usually long and tedious, and the result is not as accurate compared to the GFR test.