eGFR isn’t measured directly. It’s calculated from a simple blood test, most commonly one that checks your creatinine level. A lab formula then converts that creatinine value into an estimate of how well your kidneys are filtering blood, factoring in your age and sex. The result is reported in mL/min/1.73m², and you can think of it roughly as a percentage: an eGFR of 60 means about 60% of your kidney filtering capacity is working.
What eGFR Actually Estimates
Your kidneys contain tiny filtering units called nephrons. The glomerular filtration rate is the total volume of fluid these filters process per minute. Measuring that rate directly requires injecting a tracer substance into your bloodstream and tracking how quickly your kidneys clear it. This is accurate but impractical for routine care: it’s expensive, time-consuming, and only captures a snapshot of a few hours.
eGFR sidesteps all of that. Instead of measuring filtration directly, it uses creatinine, a waste product your muscles naturally produce and your kidneys constantly clear. When kidney function drops, creatinine builds up in your blood. The problem is that creatinine alone is a blunt tool. You can lose roughly 50% of your kidney function before creatinine rises above normal range on a standard blood panel. The eGFR formula corrects for this insensitivity by weighting the creatinine value against your age and sex, producing a much earlier warning signal.
The Blood Test Behind It
Getting your eGFR starts with a routine blood draw. The lab measures serum creatinine, then runs that number through a standardized equation. The most widely used formula today is the 2021 CKD-EPI creatinine equation, which uses your creatinine level, age, and sex. Earlier versions of this equation included a race-based adjustment, but the current version does not.
Think of eGFR as a time-averaged snapshot of your kidney function over the past few days, similar to how an HbA1c reading reflects your average blood sugar over three months. A single direct measurement of GFR, by contrast, only captures what your kidneys were doing over a few hours and misses normal daily fluctuations.
Preparing for the Test
The blood draw itself takes minutes, but preparation matters. Your provider may ask you to fast (liquids only, no solid food), avoid meat for a day or two beforehand, and hold certain medications. Cooked meat temporarily raises creatinine levels and can throw off the result, so following these instructions closely is important for accuracy.
Cystatin C: A Second Option
Creatinine isn’t the only blood marker that can estimate GFR. Cystatin C is a small protein produced by nearly all cells in your body at a steady rate, and your kidneys filter it out. Unlike creatinine, cystatin C levels aren’t significantly affected by muscle mass, making it useful in situations where creatinine-based eGFR may be unreliable.
Your provider can order an eGFR based on cystatin C alone or a combined equation that uses both creatinine and cystatin C. Research in older adults found that when the two estimates disagree, the lower of the two tends to be the more accurate one compared to directly measured GFR. Current guidelines support using cystatin C in older populations especially, since it often provides a more reliable picture in people whose muscle mass doesn’t fit typical assumptions.
When eGFR Gets It Wrong
Because creatinine comes from muscle, anything that changes your muscle mass can skew a creatinine-based eGFR. The errors are predictable and go in opposite directions depending on body type.
For every additional 10 kg of lean body mass, eGFR underestimates true kidney function by about 6 mL/min. In practice, this means a young, muscular person can get a falsely low eGFR and be incorrectly flagged as having reduced kidney function. In the highest muscle mass group studied, the test’s ability to correctly rule out moderate kidney disease dropped to just 47%. On the other end, someone who is frail or has significant muscle wasting (from aging, illness, or prolonged bed rest) will have unusually low creatinine, making their eGFR appear reassuringly normal even when their kidneys are actually struggling. Sensitivity for detecting moderate kidney disease in the most muscle-wasted group fell to 68%.
Fat tissue, interestingly, has no effect on eGFR accuracy. The issue is specifically lean mass. Other factors that can distort results include pregnancy, recent high-protein meals, certain medications that block creatinine secretion in the kidneys, and acute changes in kidney function (eGFR reflects an average, so it lags behind sudden shifts).
If your provider suspects your body composition is affecting the result, they may order a cystatin C-based eGFR or, in rare cases, a direct GFR measurement using an injected tracer.
Reading Your eGFR Number
Chronic kidney disease is classified into five stages based on eGFR, and the numbers are intuitive once you know the scale:
- Stage 1 (eGFR 90 or above): Normal filtering capacity. Kidney disease at this stage is identified by other signs, such as protein in the urine or structural abnormalities.
- Stage 2 (eGFR 60 to 89): Mildly reduced function. Often produces no symptoms.
- Stage 3 (eGFR 30 to 59): Moderate reduction. This is the stage where many people are first diagnosed, sometimes split into 3a (45 to 59) and 3b (30 to 44).
- Stage 4 (eGFR 15 to 29): Severely reduced function.
- Stage 5 (eGFR below 15): Kidney failure. At an eGFR of 15, roughly 15% of kidney function remains.
A single low reading doesn’t confirm chronic kidney disease. The result needs to be persistently reduced over at least three months. Temporary dips can happen from dehydration, illness, or medication changes.
Why eGFR Alone Isn’t the Full Picture
eGFR tells you how well your kidneys are filtering, but it doesn’t reveal whether your kidneys are leaking things they shouldn’t. That’s where the urine albumin-to-creatinine ratio (UACR) comes in. Albumin is a protein that healthy kidneys keep in the blood. When it starts showing up in urine, it signals kidney damage that may not yet affect your eGFR number.
Together, eGFR and UACR give a more complete picture. You can have a normal eGFR but an abnormal UACR, which is how Stage 1 kidney disease is often caught. If either test comes back abnormal, your provider will typically repeat it to confirm the finding before drawing conclusions.