Understanding GFR Calculation: How Your Kidney Function is Measured

Factors Affecting GFR Calculation

Glomerular filtration rate (GFR) is an important measure of kidney function, but it can be affected by a variety of factors. These factors include age, sex, race, body size, diet, and medications.

Age: GFR tends to decrease with age, starting as early as age 30 in some individuals. This is because the number of functioning nephrons (the filtering units of the kidney) decreases with age.

Sex: In general, men have a higher GFR than women. This is due to differences in body size and muscle mass.

Race: Some studies have shown that individuals of African or Asian descent may have a slightly lower GFR than individuals of European descent.

Body size: Larger individuals tend to have a higher GFR than smaller individuals, as larger individuals typically have more nephrons.

Diet: A diet high in protein can increase GFR, while a diet low in protein can decrease GFR.

Medications: Certain medications can affect GFR, either increasing or decreasing it. For example, nonsteroidal anti-inflammatory drugs (NSAIDs) can decrease GFR, while some antibiotics can increase GFR. It is important to talk to your doctor about any medications you are taking and how they may affect your GFR.

GFR Calculation Methods and Formulas

There are several ways to estimate GFR, including:

  1. Serum Creatinine-Based Equations: These equations use a blood test to measure the level of creatinine, a waste product from muscle metabolism, in the blood. The most commonly used equation is the Modification of Diet in Renal Disease (MDRD) equation, which takes into account age, sex, race, and serum creatinine level.

  2. Cystatin C-Based Equations: Cystatin C is a protein produced by all cells in the body at a constant rate and is freely filtered by the kidneys. Cystatin C-based equations use blood tests to measure the level of cystatin C in the blood to estimate GFR. These equations may be more accurate than creatinine-based equations, particularly in certain populations such as the elderly.

  3. Combination Equations: Some equations combine both serum creatinine and cystatin C levels to estimate GFR, which may improve accuracy.

It is important to note that GFR estimation equations have limitations and may not be accurate in all individuals. Additionally, GFR may change over time, so repeated measurements may be necessary to track kidney function. Your doctor can help determine which GFR calculation method is best for you and interpret your GFR results.

Interpreting GFR Results: What Do They Mean?

GFR results can help determine how well your kidneys are functioning. The normal GFR range for healthy adults is typically between 90 and 120 mL/min/1.73 m². However, GFR can vary depending on age, sex, race, and other factors.

GFR results are typically categorized as follows:

  1. Stage 1: GFR greater than or equal to 90 mL/min/1.73 m². This indicates normal kidney function.

  2. Stage 2: GFR between 60 and 89 mL/min/1.73 m². This indicates mildly reduced kidney function.

  3. Stage 3: GFR between 30 and 59 mL/min/1.73 m². This indicates moderately reduced kidney function.

  4. Stage 4: GFR between 15 and 29 mL/min/1.73 m². This indicates severely reduced kidney function.

  5. Stage 5: GFR less than 15 mL/min/1.73 m². This indicates kidney failure and may require dialysis or a kidney transplant.

It is important to note that GFR results are just one piece of information about kidney function, and other tests and factors may need to be considered when determining the overall health of your kidneys. Your doctor can help interpret your GFR results and recommend appropriate treatments or interventions to help maintain or improve kidney function.

How to Improve Your GFR and Kidney Function

There are several ways to help improve your GFR and overall kidney function, including:

  1. Manage underlying health conditions: Conditions such as diabetes and high blood pressure can damage the kidneys over time, so managing these conditions through lifestyle changes and medications can help improve kidney function.

  2. Maintain a healthy diet: Eating a balanced diet that is low in sodium and processed foods and high in fruits, vegetables, and whole grains can help support kidney health.

  3. Stay hydrated: Drinking enough water can help keep the kidneys functioning properly and prevent dehydration, which can damage the kidneys.

  4. Exercise regularly: Regular exercise can help improve overall health and support kidney function.

  5. Avoid smoking and excessive alcohol consumption: Both smoking and excessive alcohol consumption can damage the kidneys over time, so avoiding or minimizing these habits can help improve kidney function.

  6. Follow medication instructions: Taking medications as prescribed and talking to your doctor about any potential kidney-related side effects can help prevent medication-related kidney damage.

  7. Get regular check-ups: Regular kidney function tests can help detect any changes in GFR or overall kidney function and allow for early intervention if necessary.

It is important to talk to your doctor about any changes you are considering making to your lifestyle or medications to ensure they are safe and appropriate for your individual situation.


GFR is a key measure of kidney function and can help diagnose and monitor kidney disease. There are several factors that can affect GFR, including age, sex, race, body size, diet, and medications. GFR can be estimated using various equations that take into account serum creatinine levels, cystatin C levels, or a combination of both. GFR results can be interpreted as a stage of kidney disease and may require appropriate treatments or interventions. Lifestyle changes such as managing underlying health conditions, maintaining a healthy diet, staying hydrated, exercising regularly, avoiding smoking and excessive alcohol consumption, following medication instructions, and getting regular check-ups can help improve GFR and overall kidney function. It is important to talk to your doctor about any concerns or questions regarding your kidney health and GFR results.

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