eGFR Test

Your kidneys filter roughly 200 liters of blood every day, silently removing waste, balancing minerals, and regulating blood pressure. When that filtering power drops, there are no early symptoms. You will not feel it at 70% capacity. You probably will not feel it at 50%. By the time most people notice something is wrong, significant and often irreversible damage has already occurred. eGFR (estimated glomerular filtration rate) is the single best number for catching that decline early.

Unlike most blood tests, eGFR is not a molecule your body produces. It is a calculated value, derived from blood markers your kidneys are responsible for clearing. The most common version uses creatinine, a waste product from normal muscle metabolism. A newer, more accurate version combines creatinine with cystatin C, a small protein produced by nearly every cell in your body. By plugging these values into validated equations along with your age and sex, the lab estimates how many milliliters of blood your kidneys can filter per minute.

What eGFR Actually Tells You

eGFR reflects the combined work of your nephrons, the tiny filtering units inside each kidney. You are born with roughly one million nephrons per kidney, and you cannot grow new ones. As nephrons are lost to aging, high blood pressure, diabetes, or other insults, the surviving nephrons compensate by working harder. This compensation can mask the problem for years, keeping your eGFR in a range that looks acceptable even as your kidney reserve shrinks.

In healthy young adults, eGFR typically runs between 90 and 130 mL/min/1.73 m². After about age 30, eGFR naturally declines by roughly 0.8 to 1.0 mL/min per year. That gradual slide is expected. What matters is whether your decline is faster than expected, whether it is accelerating, or whether your number has crossed a threshold that signals real risk.

Heart Disease and Cardiovascular Risk

Kidney function and heart health are deeply intertwined, and eGFR is one of the strongest predictors of cardiovascular trouble. The risk relationship is not linear. It stays relatively flat above 75 mL/min/1.73 m², then climbs steeply as eGFR drops below 60.

A massive analysis of 27.5 million people across 114 global studies found that people with an eGFR between 45 and 59 had roughly 30% higher rates of hospitalization compared to those with eGFR above 90. The risk gradient was steepest for cardiovascular death and heart failure, even steeper than for heart attacks or stroke. A separate genetic analysis of over 436,000 adults from the UK Biobank confirmed this was not just correlation: genetically lower eGFR was causally linked to about 43% higher cardiovascular mortality for every 10 mL/min/1.73 m² drop.

All-Cause Mortality

The connection between eGFR and dying from any cause follows a similar pattern. In a meta-analysis of over one million participants, the risk of death stayed stable between eGFR 75 and 105 and then rose exponentially below 60. At eGFR 45, the risk of death was about 57% higher than at eGFR 95. At eGFR 15, it was roughly three times higher. These associations held after adjusting for age, blood pressure, cholesterol, diabetes, and other standard risk factors.

Source: CKD Prognosis Consortium 2010 mortality meta-analysis (Matsushita et al.)

What this means for you: an eGFR below 60 is not just a kidney problem. It is a whole-body risk marker, signaling danger across your cardiovascular system, your metabolic health, and your overall lifespan.

Kidney Disease Progression

The speed at which your eGFR declines matters as much as where it sits at any single point. An analysis of 1.7 million people found that a 30% decline in eGFR over a one to three year period was strongly associated with progression to kidney failure requiring dialysis. For someone starting at an eGFR of 35, a 30% decline carried a 64% chance of reaching end-stage kidney disease within a decade, while a 57% decline pushed that risk to 99%.

A study tracking over 12,000 adults in the Netherlands identified four distinct decline trajectories. Those whose eGFR dropped at about 4.3 mL/min per year (fast decliners) had dramatically higher mortality than those declining at the typical rate of about 0.9 mL/min per year. By age 70, the cumulative mortality for fast decliners was 68.8%, compared to 32.3% for slow decliners.

Reference Ranges

eGFR values depend heavily on age and sex. A 25-year-old and a 75-year-old should not be measured against the same yardstick. The following clinical staging system is used worldwide for chronic kidney disease (CKD) classification, but the age-specific ranges below it provide a more nuanced picture for prevention-minded readers.

These tiers are based on KDIGO 2024 guidelines. Your lab may report slightly different ranges depending on the equation used. Always compare your results within the same lab over time for the most meaningful trend.

Age-Specific Expectations

Large European and U.S. population studies have established reference values that show how eGFR shifts across the lifespan in healthy people. At age 20, the median eGFR is about 99 mL/min/1.73 m² in men and 101 in women. By age 60, the median drops to about 75 in men and 77 in women. By age 80, it falls to about 66 in men and 63 in women. These declines are expected, but if your eGFR falls below the 25th percentile for your age and sex, a 2026 study of over one million adults found that you face elevated risk of kidney failure and death, even if your absolute number is still above 60.

For younger adults specifically, research on over 1.5 million Canadians aged 18 to 39 found that even modest reductions in eGFR (below 75 mL/min/1.73 m²) were associated with higher rates of kidney failure and cardiovascular events. If you are under 40 and your eGFR is below 75, it warrants attention even though it would not trigger a CKD diagnosis by standard criteria.

The Equation Matters

Not all eGFR calculations are equally accurate. The current standard is the 2021 CKD-EPI creatinine equation, which uses only creatinine, age, and sex (no longer race). This equation gets within 30% of the true measured kidney function about 82 to 86% of the time. Adding cystatin C to the equation (the combined creatinine-cystatin C version) pushes that accuracy above 90%.

When does the combined equation matter most? When your muscle mass is unusually high or low, when you have a BMI above 40, when you are being treated for cancer or liver disease, or whenever a clinical decision hinges on your exact kidney function. If your creatinine-only eGFR lands near a diagnostic threshold (say, 58 vs. 62), confirming with the combined equation can prevent misclassification.

When Results Can Be Misleading

Your eGFR has a day-to-day variability of about 5%. That means a true eGFR of 60 could read anywhere from 57 to 63 on a given day without any real change in kidney function. A shift of 20% or more between two readings is needed before it is likely to reflect a genuine change rather than normal fluctuation.

Eating a cooked meat or fish meal within 12 hours of your blood draw can raise creatinine by about 0.23 mg/dL, which translates to roughly a 20 mL/min/1.73 m² drop in your eGFR. That is enough to push a borderline reading into abnormal territory. Fasting for at least 12 hours before testing eliminates this problem.

Muscle mass is the most persistent confounder. If you are a competitive weightlifter, your creatinine-based eGFR will underestimate your true kidney function because your muscles produce more creatinine. If you are elderly, bed-bound, or have lost significant muscle, the opposite happens: your eGFR will look reassuringly normal even when kidney function is impaired. In either case, adding cystatin C to the calculation corrects most of the error.

Acute illness, surgery, and dehydration can all transiently spike creatinine. If you are sick or recently had an operation, wait until you have recovered before interpreting an eGFR result as your baseline.

Tracking Your Trend

A single eGFR value is a snapshot. It tells you where you are right now, but it cannot tell you where you are headed. Serial measurements over time are far more valuable than any individual reading. The difference between losing 0.8 mL/min per year (normal aging) and 4 mL/min per year (fast decline) is invisible in a single test but obvious across three or four readings spaced six to twelve months apart.

Get a baseline reading now. If you are making changes (adjusting diet, starting a new medication, increasing exercise), retest in three to six months to see if the trend is moving in the right direction. After that, annual testing is a reasonable minimum for anyone tracking their health. If your eGFR is below 60 or declining faster than expected, increase to every three to six months.

Always compare results from the same lab using the same equation. Switching labs or equation versions between readings introduces noise that can look like a real change when nothing has actually shifted.