eGFR Explained: The Kidney Number on Your Blood Panel

If you have had a routine blood panel in the last decade, there is an eGFR on it. Most people skip past it. It is one of the more useful numbers on the page, and one of the more misunderstood.

What it estimates

GFR stands for glomerular filtration rate: how many milliliters of blood the kidneys filter each minute. A healthy young adult filters around 100 to 120 mL per minute. Measuring it directly requires an infusion and timed collections, which nobody does routinely.

Instead, the lab measures creatinine, a waste product of muscle that the kidneys clear at a steady rate, and plugs it into an equation with age and sex. The output is an estimate of GFR: eGFR. The "e" is doing real work in that name.

The stages

Kidney function is staged by eGFR, in mL/min/1.73m² (the unit corrects for body size).

Two things about this table are often missed.

First, G1 and G2 are not kidney disease on their own. An eGFR of 75 in a 70 year old with no protein in urine and no other findings is normal aging. Chronic kidney disease is defined as an eGFR under 60 for three months or more, or any eGFR with evidence of kidney damage, most often protein in urine.

Second, the stages are wide. Going from 88 to 62 is a large loss of function that stays inside G2. A single number matters less than the trend.

Why it is only an estimate

Creatinine comes from muscle. The equation assumes an average amount of muscle for your age and sex. Anyone far from average gets a skewed number.

• More muscle than average (athletes, bodybuilders) produces more creatinine, so eGFR reads lower than true function.
• Less muscle than average (frail older adults, people with muscle wasting, amputees) produces less creatinine, so eGFR reads higher than true function. This is the dangerous direction, because it hides real loss.
• A large meat meal or creatine supplements raise creatinine temporarily.
• Some drugs block creatinine secretion and raise the number without changing filtration. Trimethoprim and cimetidine are common examples.
• Dehydration raises it.

The equation itself has changed. The 2021 CKD EPI equation dropped a race adjustment that the earlier version included. If you compare an eGFR from 2019 to one from today, part of the difference may be the formula.

When the estimate is in doubt, labs can measure cystatin C, a different marker that does not depend on muscle, and combine it with creatinine for a better estimate.

Why it pairs with a urine test

eGFR measures how much the kidneys filter. It says nothing about whether the filters are damaged and leaking. Those are separate questions, and the second one usually goes wrong first.

The urine albumin to creatinine ratio, uACR, measures the leak. Kidney disease is staged on both axes: the G stage from eGFR and the A stage from uACR. A person with an eGFR of 95 and a uACR of 200 has kidney damage and a normal filtration rate. A person with an eGFR of 55 and a uACR of 5 has reduced filtration and no leak. Both have kidney disease. They have different risks and different treatments.

Guidelines call for both tests, yearly, in anyone with diabetes or hypertension. The blood test gets done. The urine test often does not. See why kidney disease is found late and kidney health urine tests.

What to do with your number

90 or above, no protein in urine: normal. Recheck at your routine interval.

60 to 89: ask whether a uACR was done. If not, ask for one. If both are normal, this is likely age. If protein is present, it is early kidney disease and worth treating now.

Under 60, first time: the lab will usually repeat it. Three months of results under 60 is the definition. Expect a uACR, a look at medications, blood pressure and sugar checks, and possibly an ultrasound.

A drop of more than 10 to 15 points between tests: worth asking about even if both numbers are in the normal range. Trend beats level.

Under 30: a nephrology referral is standard. This is the stage where planning ahead changes outcomes.

Home testing

There is no reliable home eGFR. It needs a blood draw and a lab. Home testing for kidney health is the urine side, uACR, which is where the earliest signal lives anyway. The blood number comes from the clinic. The urine number can come from your bathroom. Together they tell the story. Either alone tells half of it.