What Is eGFR? Understanding Kidney Function Test Results

If your blood test includes a result labelled "eGFR", it is measuring how well your kidneys are working. eGFR stands for estimated glomerular filtration rate — an estimate of how much blood your kidneys filter per minute. It is one of the most important markers of kidney health and is used to detect, stage, and monitor chronic kidney disease (CKD).

How Is eGFR Calculated?

eGFR is not measured directly. It is estimated from your blood creatinine level, combined with your age, sex, and sometimes ethnicity. Creatinine is a waste product produced by muscle metabolism. Healthy kidneys filter creatinine out of the blood efficiently; if kidney function declines, creatinine builds up in the blood and eGFR falls.

The result is expressed as millilitres per minute per 1.73 m² of body surface area (mL/min/1.73m²). A healthy young adult typically has an eGFR above 90.

What Is a Normal eGFR?

eGFR (mL/min/1.73m²)	Kidney Function	CKD Stage
90 or above	Normal or high	G1 (if other kidney damage markers present)
60–89	Mildly reduced	G2 (if other markers present)
45–59	Mildly to moderately reduced	G3a
30–44	Moderately to severely reduced	G3b
15–29	Severely reduced	G4
Below 15	Kidney failure	G5

It is important to note that an eGFR of 60–89 is only classified as CKD if there is also evidence of kidney damage — such as protein in the urine, blood in the urine, or abnormal kidney structure on imaging. An eGFR in this range without other markers is common in healthy older adults and may not indicate disease.

What Is Chronic Kidney Disease (CKD)?

CKD is defined as abnormal kidney structure or function persisting for three months or more. It is diagnosed when eGFR is below 60 mL/min/1.73m², or when there are other markers of kidney damage (such as proteinuria) alongside any eGFR level.

CKD is very common — it affects around 3 million people in the UK. Most people with early-stage CKD (stages G1–G3) have no symptoms and are diagnosed through routine blood tests. The kidneys have significant reserve capacity, and many people with mild to moderate CKD live normal lives with careful monitoring and management.

What Causes a Low eGFR?

Common causes of reduced eGFR include:

• Diabetes: The leading cause of CKD in the UK. High blood sugar damages the small blood vessels in the kidneys over time.
• High blood pressure (hypertension): The second most common cause. Uncontrolled blood pressure damages the kidney filtering units.
• Glomerulonephritis: Inflammation of the kidney filters, which can be caused by immune conditions or infections.
• Polycystic kidney disease: An inherited condition causing fluid-filled cysts to replace kidney tissue.
• Recurrent kidney infections: Repeated episodes of pyelonephritis can cause scarring.
• Obstructive causes: Conditions that block urine flow, such as an enlarged prostate or kidney stones.
• Medications: Some drugs, including NSAIDs (ibuprofen, naproxen), can reduce eGFR, particularly with long-term use.
• Acute illness: Dehydration or a severe illness can temporarily lower eGFR — this is called acute kidney injury (AKI).

What Is Proteinuria and Why Does It Matter?

Proteinuria — protein in the urine — is an important marker of kidney damage that is often checked alongside eGFR. Healthy kidneys retain protein in the blood and do not let it pass into the urine. When the kidney filters are damaged, protein leaks through into the urine.

Your test may report ACR (albumin:creatinine ratio) or PCR (protein:creatinine ratio). A raised ACR (above 3 mg/mmol) alongside reduced eGFR confirms CKD and indicates a higher risk of progression.

Does a Low eGFR Always Mean Kidney Disease?

Not necessarily. Several factors can temporarily lower eGFR or affect the calculation:

• Dehydration: Reduces blood flow to the kidneys and creatinine clearance temporarily.
• High protein diet or intense exercise: Increases creatinine production, which can lower the calculated eGFR without reflecting true kidney disease.
• Muscle mass: People with very high or very low muscle mass may have misleading eGFR estimates because creatinine production varies with muscle bulk.
• Age: eGFR naturally declines with age; an eGFR of 55 in an 80-year-old may be entirely expected.

This is why a single low eGFR result must always be repeated and interpreted in context before any diagnosis is made.

What Happens If My eGFR Is Low?

Your GP's response will depend on the level and trend of your eGFR:

• Mildly reduced (45–59): Monitoring every 6–12 months, blood pressure management, review of medications, urine dipstick for protein.
• Moderately reduced (30–44): More frequent monitoring, optimisation of blood pressure and glucose, dietary advice, referral to a nephrologist if rapidly declining.
• Severely reduced (15–29): Nephrology referral, planning for possible renal replacement therapy (dialysis or transplantation).
• Kidney failure (below 15): Urgent nephrology input; dialysis or transplantation may be needed.

How to Protect Your Kidney Function

• Control blood pressure — aim below 130/80 mmHg if you have CKD
• Control blood sugar if you have diabetes
• Stay well hydrated
• Avoid regular NSAID use (ibuprofen, naproxen, diclofenac) unless advised by your GP
• Maintain a healthy weight and exercise regularly
• Stop smoking — smoking reduces blood flow to the kidneys
• Reduce salt intake to lower blood pressure
• If prescribed ACE inhibitors or ARBs for CKD, take them as directed — they protect the kidneys even if they initially lower eGFR slightly