What is GFR (Glomerular Filtration Rate)?
Medically Reviewed by Dr. Andrew Stein MD, Consultant Nephrologist (Hospital Kidney Specialist). Last updated: June 2026
This article explains glomerular filtration rate (GFR) in plain language; what it is, why it matters, its limitations and why it should be interpreted with care.
GFR is widely used to assess kidney health, but it has important limitations that are often overlooked.
What Is Glomerular Filtration Rate (GFR)?
Glomerular filtration is the first step in urine formation and a core function of the kidneys.
Glomerular filtration rate (GFR) is defined as the volume of fluid filtered from the blood by the kidney’s glomeruli into Bowman’s capsule per unit of time.
It is measured in millilitres per minute (ml/min).
What Does GFR Tell Us?
GFR is generally accepted as the best single overall indicator of kidney filtering capacity in both healthy individuals and those with kidney disease.
- A higher GFR usually means more functioning kidney tissue – i.e. a higher number is better
- A lower GFR suggests reduced filtering ability
For this reason, GFR is commonly used as a surrogate marker of kidney function.
However, GFR does not represent total kidney function. The kidneys perform multiple tasks, and GFR reflects only one of them—filtration.
There are 6 other kidney functions, that GFR does not assess.
Is There Such a Thing as ‘Kidney Function’?
Strictly speaking, there is no single entity called ‘kidney function.’
Just as we don’t describe ‘brain function’ or ‘skin function’ with one number, kidney performance cannot be fully captured by a single measurement.
GFR measures one specific process of the kidney, not the full range of kidney activity.
Normal (average) GFR Values
In young adults, the average GFR, adjusted for body surface area, is approximately:
~110 ml/min/1.73 m²
This is an average, not a fixed normal value. But average GFR varies from 80 to 120 ml/min – depending on age and gender. This is a large research study from 2025 that looks into averages from very large European studies.
GFR is mathematically corrected to body surface area because:
- Larger bodies generally have larger kidneys
- Larger kidneys filter more fluid
Note. GFR also varies depending on which equation is used to calculate it – confusingly, there are several.
How Is GFR Measured? (eGFR Explained)
Direct measurement of GFR is complex and rarely done in routine practice. Instead, laboratories report an estimated GFR (eGFR) – based primarily on the blood creatinine level.
What is eGFR Based On?
eGFR is calculated using:
- Blood creatinine level
- Age
- Sex
- Body size
What is creatinine?
Creatinine is a waste product cleared by the kidneys. When kidney filtration declines, creatinine levels rise – so, unlike GFR, a lower number is better.
Typical adult creatinine range: ~60–120 µmol/L
The laboratory automatically calculates eGFR and reports it alongside the creatinine result.
Example
- Creatinine: 87 µmol/L
- eGFR: 112 ml/min
Both values are within the normal range, indicating normal filtering ability—but this does not rule out all kidney disease.
Why? There can be a structural problem with the kidneys, and normal kidney function.
Factors That Affect GFR
GFR naturally varies based on several factors:
- Age – GFR declines with age
- A commonly cited estimate is a fall of ~1 ml/min per year after age 40
- Sex – Higher on average in men
- Body size – Higher in larger individuals
Because of this, an older adult may have a lower GFR that reflects normal ageing, not disease.
Why GFR Must Be Interpreted Carefully
GFR values should never be interpreted in isolation.
Key Limitations of eGFR
eGFR may be inaccurate:
- In people with extreme body types (very muscular, amputees, malnourished, or morbidly obese)
- At very low GFR levels (below ~15 ml/min)
- When results are compared across different laboratories
Creatinine Must Be Stable
eGFR assumes a stable creatinine level.
Hence it is not reliable in:
- Acute kidney injury (AKI)
- Rapidly changing kidney function
- Patients on dialysis
Why GFR Is Not ‘% Kidney Function’
GFR is often mistakenly described as a percentage of kidney function. This is incorrect for two reasons:
- Filtration is only one of several kidney functions. The kidneys also regulate fluid, electrolytes, acid–base balance, blood pressure, hormones, and more.
- GFR is not based on a 100-point scale. Average normal GFR is around 110 ml/min, not 100—so expressing it as a percentage is mathematically meaningless.
Summary
- GFR measures how well the kidneys filter blood
- It is a useful but limited indicator of kidney health
- Most reported values are estimated (eGFR), not directly measured
- Age, body size, and clinical context matter
- GFR should never be interpreted as total or percentage kidney function
Used correctly, GFR is helpful. Used incorrectly, it can be misleading.
A Final (‘120’) Coincidence
- Upper limit of GFR is ~120 ml/min
- Upper limit of normal creatinine is ~120 µmol/L
Coincidence—or just a helpful number to remember?
[“Does this mean that kidney doctors are not as clever as they think they are, as they only have to remember one number?” Yes! Ed]
