What Is GFR? Understanding Glomerular Filtration Rate and Kidney Function

GFR, or glomerular filtration rate, is how much blood your kidneys filter each minute, measured through the tiny filters called glomeruli. Labs do not usually measure it directly. They estimate it (eGFR) from a blood creatinine result, your age and your sex, and report it in mL/min/1.73 m2. The NKF says the normal eGFR in adults is usually more than 90, while the NIDDK describes a GFR of 60 or more as the normal range and below 60 as a possible sign of kidney disease. Those two statements look different but describe the same scale; the sections below explain how they fit together.

What the kidneys are doing

The NIDDK says each kidney is made up of about a million filtering units called nephrons. In each one, the glomerulus filters the blood and the attached tubule returns needed substances to the blood and removes wastes. The thin walls of the glomerulus let smaller molecules, wastes and fluid pass into the tubule while larger molecules such as proteins and blood cells stay in the blood vessel. That is why protein or blood in urine can point to a problem with the filter.

Sources give different totals for daily volume. The NIDDK says the kidneys filter about 150 quarts of blood a day and that only 1 to 2 quarts become urine. The NKF says about 200 quarts of fluid a day, with about 2 quarts becoming urine. Converting to the units labs use, 150 quarts a day is about 142 litres, or roughly 99 mL/min, and 200 quarts is about 189 litres, or roughly 131 mL/min. Both are in the same range as a typical eGFR for a young adult, which tells you the figures are approximations.

Measured GFR vs eGFR

Measuring true GFR needs a specialised test using an injected filtration marker, which is why most people only ever see eGFR. The NIDDK notes that eGFR is calculated using estimating equations that may not always be precise. The creatinine-based version works like this: creatinine is a waste product that the NKF says comes from the digestion of dietary protein and the breakdown of muscle, so a higher blood level can suggest the kidneys are clearing less. Because muscle mass, diet and other factors also affect creatinine, the equation is only an estimate.

Reading the numbers: GFR categories

The NKF classifies GFR into categories G1 to G5. The categories describe filtration only; they are not a diagnosis by themselves.

Two cautions apply. The NKF states that without evidence of kidney damage, neither G1 nor G2 meets the criteria for CKD. And KDIGO 2024 defines CKD as abnormalities of kidney structure or function present for more than 3 months, either a GFR below 60 or markers of kidney damage such as albuminuria. A single low number is therefore a prompt for follow-up, not a verdict.

GFR and age

The NKF publishes average eGFR by age group for people without kidney disease. Our calculation of the step between decades is added in the right-hand column.

From the 20s to the 70s and over, the average falls by 41 points, about 35 percent. The NKF states that eGFR declines with age even in people without kidney disease. These are averages, not targets: an eGFR below the average for your age group is not automatically a problem, and one above it is not a guarantee. The 2021 CKD-EPI equation itself includes a built-in age factor of 0.9938 per year, so the same creatinine produces a lower eGFR in an older person. For a man with creatinine 1.0 mg/dL we computed 107.1 at 25, 94.6 at 45, 83.5 at 65 and 73.8 at 85. For a woman with creatinine 0.8 the values are 104.8, 92.5, 81.7 and 72.2. That is the equation applying its age term, not a measurement of how the kidneys have aged.

What can make eGFR misleading

• Muscle mass. Very high muscle mass can raise creatinine and lower the estimate; low muscle mass can have the opposite effect. The NKF lists unusual muscle mass among causes of inaccurate estimates.
• Early disease. The NKF notes eGFR can be inaccurate especially in early stages of kidney disease (stages 1 and 2).
• Pregnancy, age over 70, cirrhosis, nephrotic syndrome, a past solid organ transplant and some medicines are also listed by the NKF as reasons for possible inaccuracy.
• Diet, malnutrition and other chronic illness can affect creatinine, according to the NKF.
• Changing kidney function. Equations assume steady-state function, so results during acute illness are unreliable.

Common mistakes

• Reading one result as a diagnosis. Staging needs the trend over more than 3 months and, usually, a urine albumin test.
• Treating the age averages as cutoffs. A value below your age average is a reason to ask questions, not to assume disease.
• Comparing results from different equations. See our post on CKD-EPI vs MDRD for why numbers can shift.
• Ignoring urine tests. Protein in urine can show kidney damage even when eGFR looks normal.

Frequently asked questions

Is eGFR the same as GFR?

No. GFR is the true filtration rate; eGFR is an estimate calculated from blood creatinine, age and sex. Both are reported in mL/min/1.73 m2.

What is a normal GFR?

The NKF says the normal eGFR in adults is usually more than 90 and publishes age averages from 116 in the 20s to 75 at 70 and over. The NIDDK describes 60 or more as the normal range. Where you fall relative to those figures should be interpreted by your clinician.

Can I measure my GFR at home?

No. You can use our calculator with a creatinine value from a lab test, but the result is an estimate and not a diagnosis.