The main filtration system of your body is your kidneys. They are responsible for removing waste products and toxins from your blood and excrete these substances through your urine. Inside your kidneys are small filters called glomeruli. These filters don’t work efficiently if your kidneys are not functioning well. And that is the time when your doctor will ask you to take the glomerular filtration rate (GFR) blood test.
GFR or Glomerular Filtration Rate is the amount of blood filtered by the glomeruli measured per unit of time. It is a simple blood test to determine the current status of your kidney function. Usually, this can be ascertained through a mathematical formula wherein your sex, age, and the race is compared with your levels of serum creatinine.
The GFR test allows you to determine how well your kidneys are functioning. It evaluates the amount of blood that passes through the glomeruli every minute. Glomeruli are found in the kidneys which work as tiny filters for the waste to be removed from the blood.
This test is usually required if you are manifesting symptoms of kidney diseases. This is also required to know if a particular treatment you have undertaken is effective. Symptoms of kidney disease may include mid-back pain, foamy urine, difficulty urinating, and unexplained body swelling. This may also be done if you are taking medications for hypertension, diabetes, UTI, heart disease, kidney stones, and PCOS, among others, as these can affect your kidney function.
No special preparation is required. The GFR test procedure involves a simple blood test wherein a blood sample is extracted from your arms. To calculate GFR, you need to give the following information: age, sex, height, weight, and race.
The normal GFR in adults is more than 90 mL/min/1.73 m2. But the normal GFR declines as you age. And this will occur even if you are not suffering from certain kidney disease. The table below will show the normal estimated GFR:
| Age | Normal Estimated GFR |
|---|---|
| 20 to 29 years old | 116 |
| 30 to 39 years old | 107 |
| 40 to 49 years old | 99 |
| 50-59 years old | 93 |
| 60 to 69 years old | 85 |
| 70 years old and above | 75 |
The National Kidney Foundation states that lower GFR results may mean more damage to your kidneys. The result of your GFR test may help doctors evaluate the level of kidney damage you are suffering. This may also indicate the stage of your kidney damage. Here are the causes of low and high GFR:
| Stage | GFR | Indication |
|---|---|---|
| 1. | 90 or above | No or minimal loss of kidney function |
| 2. | 60 to 69 | Mild loss of kidney function |
| 3. | 30 to 59 | Moderate loss of kidney function |
| 4. | 15 to 29 | Severe loss of kidney function |
| 5. | 15 or below | Kidney failure |
It is worth noting that there are instances when results may vary from the numbers shown above. This is because some laboratories use different ranges. Since certain medications may affect your creatinine level, your doctor might require you to take the test to determine it.
The estimated normal value of your GFR can be obtained by deduction 140 from your age. Let’s say you are 30 years old. Your GFR is 110 ml/min (140-30). This can only be used if you are healthy. As mentioned above, your GFR is also affected by other factors such as sex, height, weight, and race. That is the reason why there are many formulas used in calculating the GFR.
Using the Cockcroft-Gault equation, you need to consider sex, age, and muscle mass. For men, you need to multiply by 1.23. So the formula for creatinine clearance is: [(140-age)x lean bodyweight x 1.23 for male]/ serum creatinine Using the MDRD (modification of diet in renal disease), the equation is equal to 186 x (Creatinine/88.4)-1.154 x (Age)-0.203 x (0.742 if female) x (1.210 if black).
One of the most prevalent diseases in the world is kidney disease. People with mild kidney damage usually suffer from a worse condition if it is uncovered and untreated over the years. That is the reason why if you suspect that you have kidney problems, it is best to take GFR blood test.
Doctor, author and fitness enthusiast, Ahmed Zayed, MD, is a surgery resident with a passion for helping people live a happy healthy life. He is the author of numerous health-related books and contributor to several medicine, health and wellbeing websites.
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