Creatinine level: what the number tells you

Creatinine is formed when muscles use creatine, the substance they use to release energy quickly. Every day about one to two percent of the creatine stored in your muscles breaks down into creatinine, which travels through the blood to the kidneys and leaves the body in the urine. The test measures how much of it stays behind in your blood, in micromoles per litre (µmol/L).

That makes the value the sum of two things: how much creatinine goes in, which depends on your muscle mass, and how much goes out, which depends on your kidneys. An ordinary report reads only the second half, because production is assumed to be constant. In someone with a lot of muscle, or with a creatine supplement in the routine, that assumption does not hold and the report reads too high.

That is why most forms also carry an eGFR, the estimated filtration rate. It is calculated from the creatinine, with a correction for age and sex, and so inherits the same error. Anyone who wants to know what their kidneys really do, independent of muscle mass, has cystatin C measured, a protein all cells release in equal measure and that has nothing to do with muscle or creatine.

The normal range differs by sex and age, mainly because of muscle mass. Men: 61 to 113 µmol/L from 19 to 50, 61 to 120 from 51 to 65, 62 to 134 from 66 to 80, and 65 to 149 above that. Women: 48 to 91 from 19 to 50, 48 to 99 from 51 to 65, 48 to 113 from 66 to 80, and 49 to 132 above that. These limits come from a Dutch study of more than seven million GP results. Dutch reports may print the substance as kreatinine; it is the same measurement.

For anyone using creatine, this is the blood value that most often causes needless alarm. Creatine is one of the best-studied sports supplements and it raises creatinine directly, because there is simply more creatine to break down. At the usual three to five grams a day the blood value usually rises by ten to thirty µmol/L; in a loading phase of twenty grams a day, more. In one careful case report the value rose from about 91 to 112 µmol/L while the actually measured kidney function stayed identical. Reviews of all the studies find no effect of creatine on measured filtration rate, on cystatin C or on protein in the urine in healthy people.

The problem is in the arithmetic. The eGFR assumes your creatinine production is average, and so reads the higher value as a slower kidney. A user with a creatinine of 118 gets an eGFR that dips below 90, and a form that suggests "mildly reduced kidney function", while nothing is wrong with the kidneys. What helps in that case is telling the doctor you use creatine, and when in doubt having cystatin C measured. More on this in high creatinine from muscle mass: what lifters need to know.

Training itself does the same on a smaller scale. A heavy session or a competition increases muscle breakdown and therefore creatinine, and the fluid loss that comes with it concentrates the blood a little further. A draw the morning after a heavy leg day or a long run therefore reads too high. A large portion of meat counts too: roasted meat contains ready-made creatinine, and a big meal makes the value rise measurably hours later.

A genuinely raised creatinine, on a clean measurement and without an explanation from muscle or supplements, means the kidneys are filtering less. That can be temporary, with dehydration, an infection or a medicine that slows excretion, such as the antibiotic trimethoprim. If it persists, the next step is a kidney work-up through the GP. How to read the four kidney values together is set out in kidney values for athletes: what creatinine and eGFR really say.

Creatinine is part of almost every general blood test. Measure it deliberately as a baseline before you start creatine, and then once a year in a quiet period; that way you can always set a later result beside your own baseline.

The circumstances of the draw decide what you measure. No heavy training in the 48 to 72 hours before, no large meat meal the evening before, and well hydrated. If you use creatine and want to know what your kidneys do without that contribution, a two-week pause before the draw gives the cleanest picture; otherwise ask for cystatin C alongside.

Is the value raised on a measurement that met those conditions? Then repeat after two to four weeks, with eGFR and cystatin C alongside. Two raised measurements in a row, without an explanation from training or supplements, are a reason to discuss the full result with your GP.