Creatinine
A muscle waste product used to gauge how well the kidneys filter.
Creatinine is a waste product. Muscle stores energy as creatine phosphate, and a small, remarkably constant fraction of that pool breaks down to creatinine every day: roughly 1 to 2% of it, whatever you happen to be doing. The kidneys filter creatinine out of the blood into the urine and barely reabsorb any of it. Put those two facts together and you have the reason it is measured: production is steady, removal is almost entirely renal, so the amount left circulating is a usable proxy for how well the kidneys are filtering.
On its own, though, the raw creatinine number is close to uninterpretable, because how much you produce depends on how much muscle you carry. A heavily built young man and a frail eighty-year-old woman with identical kidney function will post very different creatinine values. This is why laboratories report eGFR, estimated glomerular filtration rate, which feeds creatinine, age and sex into an equation to estimate filtration in millilitres per minute. The current standard is the 2021 CKD-EPI equation, which removed the race coefficient earlier versions had used, a change made because that coefficient systematically overestimated kidney function in Black patients and delayed referrals and transplant listing.
eGFR is banded into stages. Above 90 is normal filtration, 60 to 89 is mildly reduced, 45 to 59 is stage G3a, 30 to 44 is G3b, 15 to 29 is G4 and below 15 is G5, established kidney failure. Chronic kidney disease is defined as an eGFR below 60, or other evidence of kidney damage, persisting for at least three months: that duration is part of the definition, which is why a single low reading is not chronic kidney disease and is normally repeated before it means anything.
The muscle dependence cuts both ways and is the single most useful thing to understand about this marker. A very muscular person can sit above the reference range with entirely healthy kidneys. Someone with little muscle, through age, illness or inactivity, can sit comfortably inside it while having genuinely reduced function, which is the more clinically dangerous of the two errors and the harder one to spot. A low creatinine, correspondingly, almost always reflects low muscle mass rather than anything wrong; no kidney disease presents that way.
Several everyday things move it temporarily and account for a good share of unexpected results. Creatine supplements raise blood creatinine directly by enlarging the pool it comes from, without touching kidney function. A large meat meal in the hours before the draw raises it, since cooked meat contains creatinine ready-made. Hard exercise raises it briefly. Dehydration raises it. None of these are kidney problems, and all of them are worth mentioning to whoever ordered the test before a repeat is arranged.
Where the muscle confound makes creatinine unreliable, cystatin C offers a second route: it is produced by nearly all cells rather than by muscle specifically, so it is largely independent of build. Combining both markers gives a less biased estimate than either alone, which is why the creatinine-cystatin C equation is preferred when an accurate figure genuinely matters. Beyond kidney function, creatinine appears in blood-based biological-age scoring alongside albumin, the white cell count and RDW, and there, as everywhere else, the trend across several tests tells you far more than any single value.
Sources
Where your own result would sit
Most people reading this page are holding a blood-test report with this marker on it, and the question they arrived with is whether their number is normal. A reference interval says where most results from a healthy population fall; a guideline threshold is where a clinical body draws a line for a decision. Neither is a target, and a result on the wrong side of one is not a diagnosis: the same number can be unremarkable in one person and worth looking into in another, which is why the clinician who ordered the test reads it against your history rather than against a list.
Creatinine has no published interval on this page yet. We hold one, and the dashboard draws it beside a reader’s own result, but it has not been read back against the laboratory’s own page since it was collected. A figure printed beside somebody’s blood result has to have been seen at its source, not remembered, so this one waits.
On your own blood test?
Creatinine is one of the markers FitTools can read from an uploaded blood-test report. You confirm every value before anything is saved, and your results are explained together in one place.
Frequently asked questions
- What does a high creatinine mean?
- Most often it reflects how much muscle you carry rather than a kidney problem — creatinine is produced by muscle, so a muscular person reads higher with entirely normal kidneys. Creatine supplements, a large meat meal shortly before the test, hard exercise and dehydration all raise it temporarily too. Where none of those explain it, a raised creatinine and the eGFR calculated from it are the standard prompt to look at kidney function properly, which means repeating the test rather than acting on one result.
- What does a low creatinine mean?
- Nearly always low muscle mass, whether from age, illness, inactivity or simply being lightly built. It is not a sign of kidney disease — no kidney problem presents with low creatinine — and on its own it is not something to act on. Its practical importance is the reverse: it means eGFR can look reassuring in someone whose kidney function is actually reduced.
- Does creatine supplementation affect creatinine and eGFR?
- Yes, and predictably. Creatine enlarges the muscle creatine pool that creatinine is made from, so blood creatinine rises and the calculated eGFR falls, without kidney filtration having changed at all. It is a common cause of an alarming-looking result in people who lift. Tell whoever ordered the test that you supplement — it changes how the number should be read.
- What is the difference between creatinine and eGFR?
- Creatinine is the measured substance in your blood. eGFR is an estimate of how fast your kidneys are filtering, calculated from that creatinine along with your age and sex. eGFR is the more interpretable of the two because it partly corrects for the fact that bigger, more muscular people naturally produce more creatinine.
- Is one low eGFR reading chronic kidney disease?
- No. The definition requires an eGFR below 60, or other evidence of kidney damage, sustained for at least three months. A single reading below 60 is a reason to repeat the test, and a good proportion of them resolve — dehydration, a recent meal, hard exercise and short-term illness all pull the number down temporarily.
- Why did eGFR equations stop using race?
- The older equations applied a coefficient that raised the estimated filtration rate for Black patients, which systematically made kidney function look better than it was and delayed specialist referral and transplant listing. The 2021 CKD-EPI equation removed it, and where a more accurate figure is needed, adding cystatin C reduces bias further than creatinine alone.