A single glomerulus
The inside of a glomerulus showing the filter
**Glomerular filtration rate** (**GFR**) is a test used by physicians and other medical professionals to see if the kidneys are working correctly. In basic terms, it is a measurement of how much liquid and waste is passing from the blood through the tiny filters in the kidney, called the glomeruli, and out into the urine during each minute. The test measures how much creatinine is in the blood. This shows how well the kidneys are performing. In a normal healthy person the GFR stays close to the same value all of the time. The test is done by taking blood from a person and sending it to a laboratory. Normal values are between 90ml/min and 110ml/min. A value below 60ml/minute means the person has chronic renal disease and a value below 15ml/minute means the person's kidneys have stopped working.

Reasons for doing the test
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Measurement of the GFR can be very helpful to medical professionals. It helps them decide if a person has a disease of the kidneys or not. A normal value for GFR is between 90-110ml/min. This will normally go down in older people. It will also go down in people with kidney disease. As well as helping a medical professional diagnose a kidney disease, the GFR is also used to monitor the progress of a disease.

Method of testing
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The test begins by taking blood from a person's veins. The most common places are the vein in the elbow and the veins on the back of the hands. People usually feel a sharp stinging pain when the needle enters their skin. After the blood is drawn out, it is sent to a medical laboratory where a laboratory technician calculates the GFR. It is possible to calculate an exact value for the GFR but this is rarely done any more. Most of the time, the value is estimated (this is called an eGFR). The basis for this estimate is a chemical in the blood called creatinine. The calculation may also include things such as gender, age, height, weight, and race. Getting an exact measurement needs more tests and special equipment. Doctors now normally use the estimated GFR with signs, symptoms and the medical history given by the patient to come to a diagnosis. A final diagnosis of kidney disease may sometimes be made by a pathologist using tests such as a kidney biopsy.

There are some slight risks when this test is done. These include, too much bleeding from the needle entering the skin and vein, blood collecting under the skin, feeling faint, and infection from the skin being pierced by the needle.

Functions that make up the GFR
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There are many functions that make up the GFR. These include: nitrogen based waste, sodium (salt), water, potassium, phosphate, and some medicines dissolved in the plasma such as digoxin and gentamicin. Blood pressure, the balance of acids to bases, the release of erythropoietin (a chemical that tells the body to make more red blood cells). The activation of vitamin D1, the formation of glucose in the fasting state, and the creation of peptide hormones (including insulin) all affect the GFR.

Results
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A GFR of less than 60ml/minute for three or more months indicates chronic kidney disease. A GFR of less than 15ml/minute indicates full failure of the kidneys. GFR estimates between 60 and 89 mL/minute do not mean a person has chronic kidney disease unless there are other signs of disease. People with either higher than normal amounts of muscle mass, such as bodybuilders or people with lower than normal amounts of muscle mass, such as amputees or people with muscle wasting disorders (when the muscles become less dense than normal) can have GFR test results that do not appear normal, but may still be normal. A medical professional will explain the meaning of any GFR test done on a patient.

Related pages
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* Renal function
* Renal disease
* Dialysis

Other websites
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* The Kidneys and How They Work Archived 2015-05-02 at the Wayback Machine
* Glomerular filtration rate - GPNotebook Archived 2014-10-19 at the Wayback Machine