Understanding your GFR: what the number actually means
GFR is the single number most patients ask about, and also the most misunderstood. Here’s what it actually measures, what affects it day-to-day, and the more useful question to ask your nephrologist.
What GFR actually measures
Your kidneys are made of about a million tiny filters called glomeruli. GFR is an estimate of how much blood those filters clean every minute, measured in milliliters per minute (mL/min). A healthy young adult’s kidneys filter roughly 100–120 mL/min — about a soda can’s worth of blood every 60 seconds.
As you age, GFR naturally drops a little. As kidney disease progresses, it drops more. When your GFR is consistently below 60 mL/min for three months or longer, or you have other signs of kidney damage (like protein in your urine), you meet the definition of chronic kidney disease.
The CKD stages, in plain numbers
Doctors use GFR to sort CKD into stages. The boundaries are useful because they guide what we focus on at each stage — but they aren’t a countdown clock.
| Stage | GFR (mL/min) | What it usually means |
|---|---|---|
| Stage 1 | ≥ 90 | Normal function with another sign of kidney damage (often protein in urine) |
| Stage 2 | 60 – 89 | Mildly reduced — most people feel completely well |
| Stage 3a | 45 – 59 | Mild-to-moderate — most common stage we see in clinic |
| Stage 3b | 30 – 44 | Moderate-to-severe — a good time to plan ahead |
| Stage 4 | 15 – 29 | Severely reduced — we start preparing for what may come next |
| Stage 5 | < 15 | Kidney failure — dialysis or transplant becomes the conversation |
Where does your number fall?
Move the slider to your most recent eGFR and see which stage that corresponds to. Remember: a single number isn't a diagnosis — the trend over time matters more.
You're in
Stage 3a (45 – 59 mL/min)
Mild-to-moderate reduction. The most common stage we see — and very often stable for years.
- Stage 1≥ 90
- Stage 260 – 89
- Stage 3a45 – 59
- Stage 3b30 – 44
- Stage 415 – 29
- Stage 5< 15
eGFR vs. measured GFR
Almost every GFR you’ll see on a lab report is an estimated GFR — eGFR for short. It comes from a blood test called creatinine, run through an equation that adjusts for your age and sex. The most common equation in the United States is called CKD-EPI 2021. Until 2021, this equation included a race coefficient that pushed Black patients’ estimates higher; the updated version removes race entirely. If your older results look different from your newer ones, this may be why.
A measured GFR (mGFR) is a more involved test — usually involving a special marker like iohexol — and is mostly reserved for transplant evaluation, research, or unusual situations.
Why one number isn’t the whole story
Several everyday things can move eGFR up or down by 5–10 points without anything actually changing in your kidneys:
- Hydration. A dehydrated person looks like they have lower function than they do.
- A big protein meal the day before. Can transiently raise creatinine.
- Muscle mass. People with more muscle make more creatinine, which can make eGFR look slightly low.
- Some medications.ACE inhibitors and ARBs (common blood-pressure medicines) often cause a small, expected drop in eGFR when started. That drop is usually a sign the medicine is doing its job, not that it’s hurting you.
- Acute illness. A bad infection or a vomiting illness can drop eGFR briefly, then rebound.
This is why CKD is defined as findings that persist for at least three months. One low result deserves a recheck, not a panic.
The question to actually ask
Instead of “what’s my GFR?”, try asking your nephrologist:
Is my GFR stable, rising, or falling — and over what time period?
A GFR of 38 that has been 38, 39, 37, 38 for three years is a very different situation from one that was 55 a year ago. The slope— the direction and steepness of the line — is what we’re actually managing.
Frequently asked
The takeaway
Knowledge is power — not panic. Your GFR is one of many pieces of information your nephrology team uses to build a plan that fits your life. Bring your questions. We’d rather spend the time explaining than have you wondering.
This article is for education only and doesn't replace advice from your care team.
