A glomerular filtration rate (GFR) test is a simple blood test that measures how well the kidneys are functioning. A doctor may order a GFR test to gauge kidney health.
The kidneys are a pair of bean-shaped organs on each side of the spine.
In this article, we will provide an overview of GFR, including its uses, ranges, and how to interpret test results.
GFR can help indicate how well the kidneys are functioning. A doctor may order the test if they suspect symptoms related to kidney disease. This test can help a doctor estimate the amount of blood passing through the renal filters each minute.
A medical professional will calculate GFR using an estimate known as eGFR, which measures the creatinine level compared with other parameters, including age, weight, height, gender, and race.
A
- represents the excretory capacity of the kidney
- correlates directly with the kidney functioning mass
- simplifies the classification of chronic kidney disease (CKD)
- measures the risk of disease progression
- streamlines the clinical evaluation of drug dosing
Medical experts cannot directly measure GFR, but they can assess it by looking at how quickly the kidneys clear certain substances.
An mGFR test is a lengthy and complicated process that measures how quickly the kidneys can filter products that the body does not naturally produce. Alternatively, an eGFR test is a quicker process that uses a formula to help clinicians estimate how well the kidneys are working.
While mGFR is impractical and more costly, it is more accurate than eGFR and can also better identify the early stages of kidney disease. However, as mGFR is challenging for clinical practice and research purposes, eGFR is the
A 2016 article in the American Journal of Kidney Diseases notes that both measured and estimated GFRs are accurate metrics for predicting the various complications related to CKDs.
The key differences between the two methods include the following:
| mGFR | eGFR | |
|---|---|---|
| How it works | measures how well the kidneys can filter substances that the body does not produce, such as inulin and iohexol | uses a calculation to estimate how well the kidneys can filter certain substances the body produces, such as creatinine and cystatin C |
| Availability | not widely available | widely available |
| Cost | more expensive | less expensive |
| Time to complete the test | more time-consuming | less time-consuming |
| Accuracy | accurately measures GFR and can identify early stages of kidney disease | may produce inaccurate estimates of GFR, especially with early stages of kidney disease |
| Precision | capable of identifying early GFR changes | may miss early GFR changes |
A doctor may order a GFR test to assess the risk or severity of CKD. While early stage CKD is often asymptomatic, a doctor may recommend an eGFR test if the risk of developing the disease is high. CKD can occur due to a combination of factors, including ones that are physical, environmental, and social.
To help prevent late stage complications of CKD, the doctor may recommend an eGFR test for any of the following symptoms:
- changes in urinary frequency and amount
- itching
- tiredness
- swelling in the arms, legs, or feet
- muscle cramps
- nausea and vomiting
- loss of appetite
They may also order an eGFR test for someone with risk factors, which include:
- obesity
- diabetes
- high blood pressure
According to the National Kidney Foundation (NKF), the following are average eGFR measures based on age.
| Age (years) | Average eGFR |
|---|---|
| 20–29 | 116 |
| 30–39 | 107 |
| 40–49 | 99 |
| 50–59 | 93 |
| 60–69 | 85 |
| 70+ | 75 |
It is important to note that the standard eGFR number is usually more than 90. Also, eGFR declines with age, even in people without kidney disease.
A healthcare professional can determine the stage of kidney disease based on a person’s eGFR or mGFR result. According to the NKF, the results may indicate the following stages of CKD:
- Stage 1: minimal or no loss of kidney function (GFR of 90 or above)
- Stage 2: mild loss of kidney function (GFR of 60–89)
- Stage 3: moderate loss of kidney function (GFR of 30–59)
- Stage 4: severe loss of kidney function (GFR of 15–29)
- Stage 5: kidney failure (GFR of 15 or below)
A GFR result may vary slightly from the numbers listed above, depending on the testing ranges of a laboratory. To establish a pattern, a doctor may recommend taking several GFR tests over a period of months.
If a person has kidney disease, their treatment plan may include:
Lifestyle modifications
According to the United Kingdom’s National Health Service (NHS), the following doctor-recommended lifestyle changes can help improve symptoms and prevent the condition from becoming severe:
- quitting smoking
- restricting salt intake
- reducing alcohol intake
- eating a healthy, balanced diet
- engaging in regular physical exercise
Medications
The goal of medication for kidney disease is to control blood glucose, lower blood pressure, and lower cholesterol levels. As such, the doctor
Dialysis
If a person experiences kidney failure, the doctor will recommend dialysis to remove waste products and excess fluids from the blood. Treatment with dialysis will be lifelong, pending when a person can get a kidney transplant.
Kidney transplant
This is often the best alternative for people with end stage kidney failure. It involves major surgery to replace the kidney with one from a deceased or living donor. Compared with dialysis, a kidney transplant can significantly improve a person’s health outcome and quality of life.
GFR is a measure of how well the kidneys are functioning. The kidneys filter blood and waste products through tiny filters called glomeruli. A person’s standard GFR depends on their age, and it is normal for GFR to decline with age.
For a long time, mGFR was the gold standard for assessing kidney function. However, calculating kidney function with mGFR was impractical due to cost and time. Presently, medical professionals calculate the GFR using eGFR, which involves a formula to estimate kidney function.