Estimated Glomerular Filtration Rate (eGFR): Kidney Function Blood Test Explained

Key takeaways

• eGFR is calculated, not measured directly: it comes from a blood test result put through an equation.
• eGFR alone does not diagnose kidney disease: urine albumin (ACR) and clinical context are needed alongside it.
• A single low eGFR is not chronic kidney disease: CKD requires abnormality persisting beyond three months.
• Muscle mass, illness, diet and some drugs shift the number in both directions without kidney damage.
• Current equations (CKD-EPI 2021) do not use race; older MDRD and race-adjusted reports are outdated.

At a glance

Also known as

• eGFR, GFR estimate, kidney function test
• Serum creatinine with eGFR
• Renal function test (RFT) or kidney function test (KFT), eGFR is usually reported as part of these panels in India
• Commonly asked for as "creatinine test" or simply "kidney test"
• Cystatin C-based eGFR (eGFRcys), where available

What the eGFR test is

The glomeruli are the microscopic filters inside the kidneys. The true glomerular filtration rate is the volume of fluid these filters clear per minute. Measuring the true GFR requires specialised clearance studies with injected markers, which are rarely done outside research and selected clinical situations.

In everyday practice, the laboratory measures creatinine, a waste product of muscle turnover, in a blood sample, then applies a validated equation to estimate GFR. The result is standardised to a body surface area of 1.73 m?so that results are comparable between people of different sizes.

Most laboratories now use the CKD-EPI 2021 creatinine equation, which uses creatinine, age and sex and deliberately excludes race. The older MDRD equation included a race coefficient and is being phased out. Where creatinine is unreliable, a cystatin C-based eGFR or a combined creatinine-cystatin C equation gives a more dependable estimate. In children, paediatric-specific equations are used instead.

Why the eGFR test is done

• Detection of kidney disease before symptoms appear, since early CKD is usually silent.
• Staging CKD in combination with urine albumin, which determines monitoring intensity and referral.
• Tracking change over time: the trend across several results matters more than one value.
• Drug dosing and safety: many antibiotics, antidiabetic drugs, anticoagulants, anti-tubercular drugs and painkillers need dose adjustment or avoidance at reduced kidney function.
• Before contrast imaging or surgery, to plan precautions and fluid management.
• Assessing acute illness, where a rising creatinine may indicate acute kidney injury rather than chronic disease.

Who should get this test

Testing is not a general population screen for everyone. It is usually advised for:

• People with diabetes: commonly annually, or as the treating doctor advises, along with urine albumin.
• People with hypertension, heart failure, ischaemic heart disease or peripheral vascular disease.
• A family history of kidney failure, dialysis, transplant or polycystic kidney disease.
• Those with recurrent kidney stones, recurrent urinary infections, obstructive urinary symptoms or a single kidney.
• People on long-term NSAIDs, lithium, calcineurin inhibitors, certain antivirals, or prolonged courses of nephrotoxic antibiotics.
• Those starting anti-tubercular therapy or chemotherapy, where dosing depends on kidney function.
• People with unexplained anaemia, oedema, persistent frothy urine, blood in urine, or an abnormal urine routine.
• Pre-operative and pre-contrast assessment, as decided by the treating team.
• Anyone previously found to have reduced eGFR or albuminuria, for follow-up.

Whether you need the test, and how often, should be decided by your doctor based on your risk profile.

What the eGFR cannot detect or exclude

This is the part most often misunderstood. A normal eGFR is reassuring but is not a clean bill of kidney health.

• eGFR does not detect albuminuria. Significant kidney damage, including early diabetic kidney disease, can exist with a normal eGFR and abnormal urine protein. A urine albumin-creatinine ratio is a separate test.
• eGFR does not show structure. Kidney stones, cysts, tumours, scarring, hydronephrosis and obstruction are not identified by eGFR; imaging is required.
• eGFR does not give a cause. A reduced eGFR does not tell you whether the problem is diabetic kidney disease, glomerulonephritis, obstruction, drug injury or something else.
• eGFR does not reliably distinguish acute from chronic. One low value cannot separate acute kidney injury from long-standing CKD without previous results and clinical assessment.
• eGFR is insensitive to early loss of function. The remaining nephrons can compensate, so creatinine may stay within the laboratory's stated range despite real nephron loss.
• eGFR cannot exclude kidney disease. "Not identified" is not the same as "excluded". If symptoms such as swelling, frothy urine, reduced urine output or blood in urine continue despite a normal eGFR, go back to your doctor rather than assuming the kidneys are fine.
• eGFR does not assess tubular function or electrolyte handling; sodium, potassium, bicarbonate, calcium and phosphate are measured separately.

Related but different examinations

These tests answer different questions. Which one comes first depends on the clinical problem: for suspected obstruction or stone pain, imaging leads; for diabetes follow-up, eGFR with urine ACR leads; for suspected glomerular disease, urine examination and protein quantification lead. No single test is universally superior.

How to prepare

Preparation is straightforward, but instructions are operational and vary between laboratories. Your own unit's written instructions take precedence over any general advice.

• Fasting: usually not required for eGFR alone. If the eGFR is part of a panel with fasting glucose or lipids, the panel's fasting requirement applies.
• Hydration: being reasonably hydrated is sensible, and it helps the blood draw. Do not load up on large volumes of water.
• Heavy exercise: intense gym work or a long run in the preceding day or two can raise creatinine. Mention it if relevant.
• Diet: a large meat meal shortly before the sample can transiently raise creatinine.
• Supplements: tell the laboratory and your doctor if you take creatine or protein supplements, or any AYUSH, herbal or over-the-counter preparation.
• Medicines: bring your full list. Do not stop or change any prescribed medicine to prepare for this test: speak to the prescriber first.

If you have heart failure, advanced CKD or a fluid restriction

Do not increase your fluid intake before the test. Any advice to "drink plenty of water" does not apply to you. Follow the fluid limit set by your treating doctor.

If you have diabetes on glucose-lowering treatment

If fasting is required for other tests in the same panel, ask your prescriber in advance how to handle insulin or tablets on the morning of the test, and carry a source of fast-acting sugar.

In pregnancy

Kidney physiology changes substantially in pregnancy. Standard eGFR equations are not validated in pregnancy, and reporting an eGFR can be misleading. Serum creatinine itself is interpreted against pregnancy-specific expectations by the obstetric team.

In children

Paediatric equations that include height are used; adult equations should not be applied to children. Preparation for the blood draw should follow the paediatric unit's instructions.

In older adults

Filtration commonly declines with age. A modestly reduced eGFR in an older adult with no albuminuria carries a different implication from the same figure in a young adult, and is interpreted accordingly.

What happens during the test

A phlebotomist cleans the skin, applies a tourniquet and draws a small blood sample, usually from a vein in the forearm. Pressure is applied and a small dressing placed. The needle time is generally under a minute; the whole visit including registration and billing usually takes a short while, depending on how busy the collection centre is. No sedation, injection of contrast or preparation medicine is involved.

Understanding your report

The report shows serum creatinine with its laboratory reference interval, and an eGFR in mL/min/1.73 m? Laboratories often report values above a certain level simply as "greater than 90" or "greater than 60", because the equations are less precise at higher filtration rates.

Broadly, a higher eGFR indicates better filtration and a lower eGFR indicates reduced filtration. Kidney disease staging combines the eGFR category with the urine albumin category and requires the abnormality to persist for more than three months. Your doctor also weighs your age, muscle mass, medicines, current illness, blood pressure, urine findings and any previous results.

Do not stage yourself from a single number. Two people with the same eGFR can have very different outlooks, and a value slightly below the printed range is not automatically disease. Equally, a normal value alongside heavy albuminuria still signals kidney disease. Results vary depending on individual clinical circumstances, and interpretation belongs to the doctor who requested the test.

When eGFR results can be misleading

Because eGFR is calculated from creatinine, anything that alters creatinine production, handling or measurement shifts the estimate without any change in true filtration.

eGFR falsely low (creatinine falsely raised)

• High muscle mass, bodybuilding, or creatine supplement use
• A large meat-heavy meal shortly before sampling; intense exercise beforehand
• Drugs that block tubular creatinine secretion without reducing filtration, including trimethoprim (in cotrimoxazole) and cimetidine
• Dehydration, vomiting, diarrhoea or acute illness reducing kidney perfusion, here the low eGFR is real but may be temporary
• Certain assay interferences, including marked ketosis or very high bilirubin, depending on the method used

eGFR falsely high (creatinine falsely lowered)

• Low muscle mass: frailty, prolonged illness, malnutrition, limb amputation, spinal cord injury, muscular dystrophy
• Advanced liver disease, where creatinine generation falls and fluid retention dilutes it
• Vegetarian or very low-protein diets, relevant across much of India
• Pregnancy and early stages of some conditions with increased filtration
• These situations can make kidney function look better than it is; cystatin C-based estimation is often preferred

Situations where any eGFR equation performs poorly

• Rapidly changing kidney function, as in acute kidney injury, creatinine lags behind the real change
• Extremes of body size or composition
• Pregnancy, and people on dialysis
• Dose calculation for narrow-margin drugs, where measured or non-indexed estimates may be needed

Risks and safety

Risk from the blood test itself

Venepuncture is a low-risk procedure. Possible effects include brief pain, bruising, a small haematoma, light-headedness or fainting, and rarely local infection or nerve irritation. Tell the phlebotomist if you have a bleeding disorder, are on blood thinners, or have fainted during blood tests before.

Risk from misinterpretation

The greater risk with eGFR is not the needle but the number. An over-read result can trigger unnecessary anxiety and tests; an under-read result can delay treatment or lead to unsafe drug dosing. A single value in isolation, without urine testing and clinical review, is the most common source of error.

Special situations

Pregnancy

Standard eGFR equations are not validated in pregnancy and many laboratories suppress the eGFR on reports for pregnant patients. Kidney assessment in pregnancy relies on creatinine trends, blood pressure and urine protein, interpreted by the obstetric team.

Children and adolescents

Paediatric estimating equations include height and differ from adult formulae. Normal values also change with age in infancy and early childhood, so adult categories do not apply.

Older adults

A gradual decline in filtration is common with age. Management focuses on avoiding nephrotoxic drugs, correct dosing, blood pressure control and monitoring, rather than on the label attached to the number.

Diabetes on glucose-lowering therapy

Several diabetes medicines require dose review as eGFR falls, and some are chosen specifically for kidney protection. Any change must come from your prescriber, never from reading your own report.

Chronic kidney disease

In established CKD, the direction and speed of change across serial results matter more than any single figure. Keep your old reports so the trend can be assessed.

Heart failure

eGFR often fluctuates with diuretic dose and fluid status in heart failure. A modest change may reflect fluid shifts rather than new kidney damage, and needs review by the treating team before medicines are altered.

Liver disease

Creatinine-based eGFR frequently overestimates kidney function in cirrhosis. Cystatin C-based estimation is often more informative here.

Kidney donors and recipients

Donor evaluation and transplant follow-up typically use more precise assessment, which may include measured GFR studies, rather than routine creatinine-based estimates alone.

eGFR in the Indian context

• Diabetes and hypertension are the leading contributors to chronic kidney disease in India, which makes regular eGFR plus urine albumin testing in these groups particularly important.
• Metabolic disease occurs at lower BMI in South Asians, so kidney risk should not be dismissed in someone who looks slim.
• Race coefficients do not apply. The older MDRD "African American" adjustment was never appropriate for Indian patients, and the CKD-EPI 2021 equation removes race entirely. If your report mentions a race-based correction, ask the laboratory which equation was used.
• Vegetarian and low-protein diets are common and lower creatinine generation, which can make eGFR look better than the true filtration rate.
• Over-the-counter painkillers, unregulated herbal and gym supplements are a recognised and avoidable cause of kidney injury. Declare everything you take.
• Anti-tubercular therapy and several antibiotics need dose adjustment according to kidney function; baseline eGFR matters before starting.
• Dehydration during heat waves and diarrhoeal illness is a frequent trigger for acute kidney injury, especially in outdoor workers and older adults.
• Laboratory standardisation varies. Compare results from the same laboratory where possible, since assay calibration and the equation used can differ between centres.
• Data protection: reports and any online forms are handled under the Digital Personal Data Protection Act, 2023.

Cost and insurance in India

eGFR is almost never billed on its own. It is reported as a calculated value along with serum creatinine, or within a renal or kidney function panel, at no separate charge for the calculation itself.

Factors that influence what you pay:

• Whether you order serum creatinine alone, an RFT/KFT panel, or a wider health check package
• Whether cystatin C is added, which is a specialised and considerably costlier assay with limited availability
• Whether urine albumin-creatinine ratio is included
• Government, trust, corporate or private laboratory setting, and the city
• Home sample collection charges, urgent or stat processing, and repeat testing frequency
• Whether the test is part of an admission, day-care or pre-operative workup

Insurance behaviour: investigations done during an admissible hospital admission or approved day-care procedure are usually covered as part of the claim. Standalone outpatient pathology and preventive health checks are covered only if your policy includes an OPD or health-check benefit. Check with your insurer or the hospital's insurance desk before testing.

Myths and facts

Frequently asked questions

Is eGFR the same as a creatinine test?

No. Serum creatinine is the laboratory measurement. The eGFR is a number calculated from that creatinine along with age and sex, standardised to body surface area. The eGFR is easier to interpret than raw creatinine, but it is still an estimate rather than a direct measurement of filtration.

How long does the eGFR process take?

The blood draw itself usually takes about a minute, with a short visit for registration and sampling. The eGFR is generated automatically alongside the creatinine result, so no extra processing time is involved. Reporting time depends on your laboratory's workflow and whether the sample is marked urgent.

Do I need to fast before an eGFR test?

Fasting is usually unnecessary for eGFR. If the test is bundled with fasting glucose, lipids or an insulin study, the panel's fasting requirement applies. Avoid a heavy meat meal and intense exercise just before the sample, since both can transiently raise creatinine. Follow your laboratory's instructions.

My eGFR is 75. Should I be worried?

Not necessarily, and not from one value. A mildly reduced eGFR needs repeating after an interval your doctor advises, plus a urine albumin-creatinine ratio and blood pressure check. Age, muscle mass and recent illness all influence the figure. Your doctor interprets the result in your clinical context.

Can eGFR improve?

Yes, in some situations. If the fall was caused by dehydration, an acute illness, a painkiller or another reversible factor, the eGFR often recovers once the cause is addressed. Established structural damage is usually not reversible, though progression can often be slowed. Results vary by individual.

Why did my eGFR change between two laboratories?

Creatinine assays are calibrated differently, laboratories may use different equations, and biological variation plus hydration status add further movement. Small differences are expected. For monitoring, use the same laboratory where possible and compare the trend rather than isolated figures.

What is cystatin C and when is it used?

Cystatin C is an alternative blood marker of filtration that is much less affected by muscle mass and diet. Cystatin C-based eGFR is useful in frailty, amputation, high muscle bulk, liver disease, or when the creatinine result appears inconsistent with the clinical picture. Availability and cost are limiting factors.

Is a home test kit enough to know my eGFR?

Home creatinine devices are not a substitute for a standardised laboratory assay, and eGFR requires a calibrated creatinine value plus correct demographic inputs. Use a laboratory test ordered and interpreted by a doctor, especially if any treatment decision or drug dose depends on it.

Does eGFR tell me why my kidney function is reduced?

No. The eGFR quantifies filtration only. Establishing a cause needs urine examination and albumin quantification, blood tests for diabetes and immune causes, ultrasound of the kidneys and urinary tract, review of all medicines, and occasionally a kidney biopsy.

Sources

• KDIGO Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease (2024), Kidney Disease: Improving Global Outcomes.
• Inker LA, Eneanya ND, Coresh J, et al. New creatinine- and cystatin C-based equations to estimate GFR without race. New England Journal of Medicine, 2021.
• National Kidney Foundation and American Society of Nephrology Task Force on Reassessing the Inclusion of Race in Diagnosing Kidney Disease, final report, 2021.
• Indian Society of Nephrology, clinical practice recommendations on chronic kidney disease.
• National Programme for Prevention and Control of Non-Communicable Diseases, Ministry of Health and Family Welfare, Government of India.
• Digital Personal Data Protection Act, 2023, Government of India.