Have a question? GFR is short for Glomerular Filtration Rate, which is the flow rate for filtered fluids through our kidney, a general measure of the kidney’s effectiveness.
Our calculator will require that you enter your serum creatinine in mg / dL or micromole / L, your age in year, your gender, and your race.
What Is GFR?
As we stated, the Glomerular Filtration Rate is about the flow rate for filtered fluids in the kidney and is used to see how well the kidney functions.
When the GFR is higher, it is counted as a better and healthier result. Large decreases in the functioning of the kidney could indicate that the organ is not functioning correctly, resulting in a lot lower fluid volume being filtered over time.
A normal range for GFR will be around 100 – 130 mL per minute per 1.73m ^2 for men; for women under 40, it is 90 – 120 mL per minute per 1.73 ^2.
After the age of 40, there is a notable decrease that occurs. Let’s take a look at the mean GFR for the average age groups in the population.:
- ages 20 – 29 average is 116 mL per minute per 1.73 m ^2
- ages 30 – 39 average is 107 mL per minute per 1.73 m ^2
- ages 40 – 49 average is 99 mL per minute per 1.73 m ^2
- ages 50 – 59 average is 93 mL per minute per 1.73 m ^2
- ages 60 – 69 average is 85 mL per minute per 1.73 m ^2
- ages 70 and over average is 75 mL per minute per 1.73 m ^2
How Do We Measure GFR?
The best way for us to measure someone’s GFR is by measuring the rate of insulin leaving the body through the bloodstream.
This is the best way to test GFR as insulin is filtered freely through the kidney and has no stage where it goes through metabolization, secretion from tubes, or absorption. This makes it the optimal candidate for measuring the filtration rate.
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Proper measurement will require a continuous intravenously infusion, along with many blood and urine collections, to test the insulin present.
It will also require that the blood and urine samples are properly timed. Due to all of these conditions needing to be met, it is not truly a test we can test very often.
This is why we have formulas created that allow us to test and estimate this result using other things. The chosen way to test this is through creatinine clearance, or estimations of creatinine clearance, based on serum levels of creatinine.
The clearance of creatinine has an important limitation to note, which is that it is affected by muscle mass.
There will be some significant changes when high levels of muscle mass are not accounted for. So there will be some variations in the effectiveness of these formulas when there are significant changes in the amount of muscle mass.
An overestimation is present in the realm of 10 – 20% higher than what GFR should be.
But, with this major issue, getting a reliable GFR estimate is still easy. Let’s look at the formulas that are used for GFR.
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GFR Formulas
The CKD- EPI formula is the main formula used to find GFR in this calculator. Here is how that looks for each population group:
Black female
- If SCr < 0.7 then GFR = 166 × (SCr/0.7)^-0.329 × 0.993^age
- If SCr > 0.7GFR = 166 × (SCr/0.7)^-1.209 × 0.993^age
Black male
- If SCr < 0.9GFR = 163 × (SCr/0.9)^-0.411 × 0.993^age
- If SCr > 0.9GFR = 163 × (SCr/0.9)^-1.209 × 0.993^age
Non-Black female
- If SCr < 0.7GFR = 144 × (SCr/0.7)^-0.329 × 0.993^age
- If SCr > 0.7GFR = 144 × (SCr/0.7)^-1.209 × 0.993^age
Non-Black male
- If SCr < 0.9GFR = 141 × (SCr/0.9)^-0.411 × 0.993^age
- If SCr > 0.9GFR = 141 × (SCr/0.9)^-1.209 × 0.993^age
The next equation that I often use is the IDMS-traceable MDRD study equation. This looks like this:
- GFR = 175 × (SCr)-1.154 × (age)-0.203 × (0.742 if female) × (1.212 if Black) (where SCr = serum creatinine in mg/dL)
The third equation is the Mayo Quadratic Formula. This looks like this:
- If SCr < 0.8 mg/dL, use 0.8 mg/dL for SCrGFR = e(1.911 + 5.249/SCr – 2.114/SCr2 – 0.00686 × age – (0.205 if female))
The final equation is specifically for children; it is the Schwartz formula:
- GFR = 0.413 × height(in cm) / SCr
Can GFR Predict Chronic Kidney Disease?
It is entirely possible to see the many stages of chronic kidney disease by looking at the GFR of a person.
Any GFR reading higher than 60 mL per minute per 1.72m^2. It is considered to be ok for the general population.
The above number will not necessarily show that a person has chronic kidney disease, but if it is partnered with a large decline from former tests, it could be an indicator.
Let’s look at the common stages of chronic kidney disease to find out what numbers to be aware of:
- Normal values are 90 or higher without proteinuria.
- Chronic kidney disease stage 1 is shown with the same 90 or higher GFR value with some evidence of kidney damage.
- Chronic kidney disease stage 2 is shown by 60 – 89 value GFR with evidence of some kidney damage.
- Chronic kidney disease stage 3m is shown by a 30 – 59 GFR value.
- Chronic kidney disease stage 4 is shown by a 15 – 29 GFR value.
- Chronic kidney disease stage 5 is shown by a 15 or lower GFR value.
We hope that this page has helped you in your fitness and wellness journey.
Make sure to check out our other calculators on our site, PTPioneer.
Tyler Read
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