Understanding Your Kidney Function Test: eGFR, Creatinine and Stages

An eGFR result estimates how many millilitres of blood your kidneys clear of waste each minute. It is not measured directly — doing that requires infusing a marker and sampling over several hours, which is accurate and almost never done outside research.

Instead it is inferred from creatinine, a waste product of normal muscle turnover that healthy kidneys clear steadily. If filtration falls, creatinine accumulates in the blood. The equation runs that logic backwards, adjusting for age and sex because both change how much creatinine a person produces in the first place.

This is the crucial thing to hold onto: creatinine is a proxy, not a measurement. Anything that changes how much of it your body makes will move the estimate without touching your actual kidney function.

Every eGFR result is reported per 1.73 square metres of body surface area. That figure is not yours — it is a standard adult body, chosen in the 1920s as the average for young adults, and it exists so that a large person and a small person can be compared on one scale.

For most purposes that normalisation is exactly what you want. It becomes relevant when you are far from average size, because your kidneys' actual filtration rate then differs from the reported one. Some drug dosing uses the un-normalised figure for precisely that reason.

You can see your own figure with the Body Surface Area Calculator, and the GFR Calculator shows the Cockcroft–Gault creatinine clearance alongside eGFR when you enter a weight.

The KDIGO staging system runs from G1 to G5, and it is routinely misread. G2 — an eGFR between 60 and 89 — appears on a great many results belonging to entirely healthy people, particularly with age, and on its own it is not a disease.

Chronic kidney disease requires two things that a single number cannot supply. The change has to have persisted for at least three months, and stages G1 and G2 require a separate marker of kidney damage, most often albumin leaking into the urine.

That second axis is why your clinician may order a urine albumin-to-creatinine ratio alongside the blood test. Albuminuria frequently appears before eGFR falls, so a normal filtration rate with raised albumin is a more concerning picture than the reverse.

Because creatinine originates in muscle, the estimate systematically misleads at both extremes of muscle mass — and it misleads in opposite directions, which is what makes it easy to miss.

A heavily muscled person produces more creatinine, so their eGFR reads low despite healthy kidneys. Someone with little muscle — an older adult, a person with a muscle-wasting condition, an amputee — produces less, so their eGFR reads high and can mask genuine impairment. The second error is the more dangerous one, because it is reassuring.

Several everyday things move it too: a large meat meal, creatine supplementation, and medications such as trimethoprim and cimetidine, which block creatinine secretion without affecting filtration at all.

Cystatin C is a small protein produced by all nucleated cells at a fairly constant rate and cleared by the kidneys. Because its production does not depend on muscle, it fails under different conditions than creatinine does — which is exactly what makes the pair useful together.

Current KDIGO guidance recommends confirming a creatinine-based eGFR with cystatin C whenever the result would change what happens next: a diagnosis, a referral, a transplant assessment, or a drug dose. It is especially worth asking about if you are unusually muscular or unusually frail.

When both markers are available, the combined equation is more accurate than either alone, and it is what the calculator leads with.

If you have an older result to compare against, you may find the numbers do not line up, and the reason is worth knowing.

Until 2021, eGFR equations included a coefficient that raised the reported figure for Black patients. It came from an average difference in serum creatinine observed in the original development cohorts — most plausibly a difference in average muscle mass — that was encoded as though race were a biological variable.

The consequence ran one way: a higher reported eGFR looks like better kidney function, which meant later referral, later diagnosis, and later placement on transplant waiting lists. In 2021 a joint task force of the National Kidney Foundation and the American Society of Nephrology recommended a refit equation with race removed entirely, and that is the equation used here.

Some causes of a low reading are reversible — dehydration, an acute illness, a medication — and those recover. Nephrons lost to chronic damage do not come back. What is very much modifiable is the rate of further loss, and the interventions with the strongest evidence are unglamorous.