Estimated Glomerular Filtration Rate (eGFR) Blood Test

What Does This Test Measure?

eGFR estimates how much blood the kidneys filter per minute, calculated from serum creatinine, age, and sex using the CKD-EPI equation. It is the primary metric for detecting, staging, and monitoring chronic kidney disease (CKD).

Why Your Doctor Ordered This Test

Doctors order eGFR primarily to screen for chronic kidney disease in people with diabetes, high blood pressure, cardiovascular disease, family history of kidney failure, or over age 60. It is also used to monitor known CKD to track progression, guide medication dosing (especially for drugs cleared by kidneys such as metformin, NSAIDs, chemotherapy), and assess risk before contrast imaging or surgery. In acute settings, a sudden drop in eGFR may indicate acute kidney injury from sepsis, dehydration, or nephrotoxic drugs, requiring immediate intervention.

The test provides an estimate of the percentage of normal kidney function and allows staging of CKD from Stage 1 (≥90 ml/min with damage) to Stage 5 (<15 ml/min, or kidney failure). A result persistently below 60 ml/min for three months confirms CKD. Values below 45 prompt referral to a nephrologist for management of complications like anemia, bone disease, and electrolyte imbalances. eGFR also helps predict cardiovascular risk, as declining kidney function independently increases heart attack and stroke risk.

What High Estimated Glomerular Filtration Rate Means

eGFR above 90 is normal. Values above 120 can occur in pregnancy, high muscle mass, or early diabetic hyperfiltration. Some labs report ">60" rather than exact values when function is normal.

What Low Estimated Glomerular Filtration Rate Means

Low eGFR (<60 for ≥3 months) defines CKD. Causes: diabetes (leading cause), hypertension, glomerulonephritis, polycystic kidney disease, recurrent infections, prolonged NSAID use, and aging. Acute drops suggest acute kidney injury from dehydration, sepsis, nephrotoxic drugs, or obstruction. A single low value should be confirmed with repeat testing.

How to Prepare and What Affects the Result

Because eGFR is calculated from creatinine, factors that alter creatinine levels can distort the result. Traditionally, a fasting blood sample is recommended, as a large meat meal can transiently raise creatinine by 10–20% due to creatine in muscle. Avoid heavy exercise for 24 hours before the test; intense weightlifting or marathon running can temporarily increase creatinine from muscle breakdown. Stay normally hydrated; dehydration concentrates the blood and lowers eGFR values artificially. Inform your doctor about all medications and supplements. Drugs like cimetidine, trimethoprim, salicylates, and fenofibrate can block creatinine secretion in the kidney tubules, causing a rise in blood creatinine and a falsely low eGFR without true kidney damage. Similarly, large doses of vitamin C can interfere with the creatinine assay in some labs. If you take creatine supplements for bodybuilding, they can significantly elevate creatinine and underestimate eGFR. The test should ideally be repeated on two separate occasions at least 3 months apart to confirm chronic disease, and any acute illness can temporarily reduce eGFR, so testing when well is preferred.

Common Misinterpretations

A common error is interpreting a single low eGFR value as definitive chronic kidney disease without confirming persistence over three months. Acute drops caused by dehydration, infection, or medications like NSAIDs can temporarily reduce eGFR, and only repeated testing can differentiate AKI from CKD. Another mistake is assuming an eGFR above 90 rules out kidney disease entirely; significant kidney damage can exist with normal filtration if there is albuminuria, as in stages 1 and 2 CKD, which require urine screening to detect. The test is also less accurate in extremes of body composition. Very muscular individuals may have a low eGFR due to high creatinine production, while those with sarcopenia, amputations, or muscle-wasting diseases may have a falsely high eGFR, masking true renal impairment.

In pregnancy, the glomerular filtration rate normally increases by 50%, so a “normal” eGFR of 70–80 may actually indicate impaired function, while a result of 95 could be falsely reassuring. Early diabetic nephropathy often presents with hyperfiltration (eGFR >120), which may be mistaken for excellent kidney health. Additionally, the long-used practice of applying a race coefficient to eGFR calculations overestimated GFR in Black patients by up to 16%, potentially delaying dialysis referral; many labs now report a single race-free equation. Finally, eGFR is not accurate in patients with acutely changing creatinine levels, such as those in the ICU, where timed urine collections or cystatin C may be needed.

Frequently Asked Questions

What does eGFR mean in a blood test?

eGFR stands for estimated glomerular filtration rate. It calculates how many milliliters of blood your kidneys filter each minute, adjusted for body surface area. The number is derived from your creatinine level, age, biological sex, and sometimes cystatin C. It is the standard way to assess kidney function and stage chronic kidney disease, with a value above 90 considered normal unless other signs of damage are present.

What is a dangerous eGFR level?

An eGFR persistently below 60 mL/min/1.73m² for three months or longer signals chronic kidney disease and should be evaluated. Values below 45 indicate stage 3b CKD, where complications like anemia and bone disease become more common, and referral to a nephrologist is recommended. An eGFR under 15 defines stage 5 kidney failure, which usually requires dialysis or a kidney transplant. Even mildly reduced eGFR increases cardiovascular risk.

Can dehydration affect eGFR?

Yes, dehydration reduces blood volume reaching the kidneys, which can temporarily lower eGFR. The test relies on creatinine, which becomes more concentrated when fluid intake is low. A single dehydrated reading may mimic chronic kidney disease, so doctors usually recheck eGFR after rehydration. Staying well-hydrated for 24 hours before testing helps ensure an accurate result. If your eGFR returns to normal after fluids, acute dehydration rather than permanent kidney damage is likely the cause.

How can I improve my eGFR naturally?

eGFR reflects kidney function, which declines naturally with age and damage, but you can slow further loss by controlling blood pressure (below 130/80 mmHg), managing diabetes tightly, reducing salt intake, staying hydrated, and avoiding nephrotoxic drugs like high-dose NSAIDs. A plant-focused diet low in processed foods may reduce kidney workload. Exercise and weight management also help. However, any improvement in eGFR after an acute drop is typically from resolving the temporary cause, not from reversing chronic scarring.

What is the difference between eGFR and creatinine?

Creatinine is a waste product from normal muscle breakdown that the kidneys filter from the blood. eGFR is a calculation that uses the creatinine blood level along with age, sex, and sometimes body size to estimate the kidney filtration rate. Creatinine alone can be misleading because it varies with muscle mass; a bodybuilder and a frail elderly person could have the same creatinine but very different kidney function. eGFR adjusts for these factors to provide a more functional picture.

Related Markers: Read Together for Full Context

BloodWorker reads lab values together, not in isolation. These related markers provide essential context for interpreting your Estimated Glomerular Filtration Rate result: