What Is GFR? - Blood Test, Levels & Calculation

Instructor: Meghan Greenwood

Meghan has taught undergraduate and graduate level science courses and has a PhD in Immunology.

In this lesson, you will learn about GFR; what it measures as well as what levels are considered normal in a healthy human. The blood test and calculations used to determine GFR will also be described.

Main Functions of the Kidney

If you've had the unfortunate experience of suffering through a kidney infection, or passing a kidney stone, you may think that the kidney doesn't do much for the body besides control urination and inflict pain. However, the laundry list of functions the kidneys have is not only impressive, but essential to living a healthy life. True, the kidneys are responsible for excreting waste, but they also regulate your body's electrolytes, acidity, red blood cells, blood pressure, and glucose production. Basically, they are two filters located in your lower abdomen, tucked behind the lowest ribs (as shown in the image below), that heavily coordinate with your heart. Although you can live with only one kidney, maintaining kidney health, by watching your diet and controlling any illness, is imperative.

kidney position


Your kidneys are composed of an intricate highway of vessels, tubes, and filters. There are plenty of traffic lights within this highway that stop certain substances from going back into the bloodstream, while allowing others to go freely. Each kidney contains close to 1 million nephrons, which are the basic functional units that filter waste and excess fluids from the blood, producing urine. The nephrons themselves are made up of tiny blood vessels called capillaries, and a cluster of these capillaries, located at the end of every nephron, is called a glomerulus. Similar to the shape of a lollipop, the glomerulus looks like a ball at the end of a large tube that excretes urine (see image below). The glomerulus filters the waste from your blood. The labels on the image below highlight the main portions of the nephron, but will not be discussed further for the purpose of this lesson.


Glomerular Filtration Rate

If your doctor suspects that your kidneys may not be functioning at 100%, he may order a blood test to measure your glomerular filtration rate or GFR. Specifically, the GFR is a calculation of how much blood is filtered through the kidneys over time. The main blood test component used to calculate GFR is serum creatinine. Creatinine is produced from the muscles and most, if not all, is excreted by the kidneys. Therefore, if the levels of creatinine increase in your blood, it is likely an indication that your kidneys are not properly filtering waste. The GFR takes this level into account, along with age, gender, and sometimes, race. Normal GFR is between 90-120 milliliters/minute.

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