An eGFR test stands for estimated Glomerular Filtration Rate. It is a blood test used to check how well your kidneys are functioning by estimating how much blood passes through the tiny filters in your kidneys, called glomeruli, each minute. This calculation provides a clear picture of your kidney health and is the standard tool for diagnosing and staging chronic kidney disease.
What We Will Cover in This Article
- The clinical definition of eGFR and why it is used.
- How the test is calculated using creatinine, age, and biological sex.
- Understanding the different stages of kidney function based on results.
- Factors that can influence the accuracy of an eGFR reading.
- Why regular testing is vital for individuals with high blood pressure or diabetes.
- The difference between measured GFR and estimated GFR.
- Next steps to take if your result falls below the normal range.
How the eGFR Test Works
The eGFR test is primarily based on a blood sample that measures the level of creatinine, a waste product produced by the natural breakdown of muscle tissue. Healthy kidneys filter creatinine out of the blood and into the urine. If your kidneys are not working effectively, creatinine builds up in your bloodstream. Because muscle mass remains relatively stable, a rise in creatinine is often a reliable indicator of declining kidney function.
To provide a more accurate estimate than creatinine alone, clinicians use a mathematical formula. This formula takes your creatinine level and adjusts it for your age and biological sex. In the UK, the NHS typically uses the CKD-EPI formula to provide this estimate. A higher eGFR number indicates better kidney function, while a lower number suggests that the kidneys are struggling to clear waste.
- Filtration Speed: Measures the volume of blood cleaned per minute.
- Calculation Factors: Uses serum creatinine, age, and sex for the estimate.
- Routine Screening: Often part of a standard ‘Urea and Electrolytes’ (U&E) blood panel.
Interpreting Your eGFR Results and Stages
Your eGFR result is expressed in millilitres per minute per 1.73 square metres (ml/min/1.73m^2). For most adults, a result of 90 or above is considered to be within the normal range, provided there are no other signs of kidney damage like protein in the urine. If your result stays below 60 for more than three months, it usually confirms a diagnosis of chronic kidney disease.
The NHS categorises kidney function into five stages. These stages help your clinical team determine the level of monitoring and treatment you may require.
| eGFR Stage | Result (ml/min/1.73m2) | Meaning |
| Stage 1 | 90 or above | Normal or high function (with signs of damage) |
| Stage 2 | 60 to 89 | Mildly decreased function |
| Stage 3a | 45 to 59 | Mild to moderate decrease |
| Stage 3b | 30 to 44 | Moderate to severe decrease |
| Stage 4 | 15 to 29 | Severe decrease in function |
| Stage 5 | Below 15 | Kidney failure |
Factors That Influence eGFR Accuracy
While the eGFR is a highly useful tool, it is still an ‘estimate’. Certain factors can make the result less accurate for some individuals. For example, people with high muscle mass, such as bodybuilders, may have naturally higher creatinine levels, which can lead to a falsely low eGFR. Conversely, individuals with very low muscle mass, such as those with certain muscle-wasting conditions, may have a falsely high eGFR.
Other temporary factors like dehydration, a recent high-protein meal, or intense exercise can also affect the creatinine levels in your blood. This is why doctors often look for a trend over several tests rather than relying on a single one-off reading.
- Muscle Mass: Higher muscle mass can lower the estimated score.
- Pregnancy: Kidney filtration rates naturally change during pregnancy.
- Acute Illness: Infections or dehydration can cause temporary dips in function.
To Summarise
The eGFR test is an essential diagnostic tool that estimates how well your kidneys are filtering waste from your blood. By using a blood creatinine level and adjusting for age and sex, clinicians can identify kidney issues long before physical symptoms appear. Understanding your eGFR stage allows you to work with your healthcare team to implement lifestyle changes and treatments that protect your renal health for the long term.
If you experience severe, sudden, or worsening symptoms, call 999 immediately.
Is the eGFR test the same as a creatinine test?
The creatinine test provides the raw data, which is then used in a formula to calculate your eGFR score.
Do I need to fast before an eGFR test?
Fasting is not usually required, but you may be asked to avoid eating cooked meat for 24 hours before the test for the most accurate result.
What does a low eGFR mean for me?
A low result suggests your kidneys are not filtering as well as they should; your doctor will likely repeat the test to see if it is a persistent issue.
Can my eGFR improve?
If the cause is an acute issue like dehydration or a temporary infection, the score can improve as you recover.
Why is age included in the eGFR formula?
Kidney function naturally declines slowly as we get older, so the formula accounts for these expected age-related changes.
How often should I have an eGFR test?
If you have risk factors like diabetes or high blood pressure, an annual test is standard, but those with known kidney issues may need them more frequently.
Authority Snapshot
This article explains the clinical importance and interpretation of the eGFR test, following established NHS and UK medical frameworks. Dr. Rebecca Fernandez, a UK-trained physician with experience in internal medicine, cardiology, and emergency care, has reviewed this content. Her expertise in interpreting diagnostic results and managing acute renal risks ensures that this information is accurate and emphasizes the role of clinical testing in maintaining kidney health.
