Creatinine value and your kidney function value explained

Your muscles use a substance called creatine to deliver power quickly, and that use produces a little waste every day: creatinine. That waste travels through your blood to your kidneys, which get rid of it in the urine. The lab measures how much of it is still in your blood, in micromoles per litre (µmol/L). If your kidneys work well, little is left over; if they work less, it builds up. Dutch reports may print kreatinine; it is the same thing.

There is one thing you need to know to read the number: it also depends on how much muscle you have. More muscle is more creatinine, regardless of your kidneys. That is why the normal range for men is higher than for women, and why a muscular person can sit above the limit with kidneys that work perfectly well.

To get around that, the lab calculates a second number: the eGFR, your kidney function value. It estimates how many millilitres of blood your kidneys filter per minute, corrected for your age and sex. Above 90 is normal, and that number is the first one a doctor looks at. If the doctor is unsure because you have a lot of muscle or use creatine, there is cystatin C, a value that does not depend on muscle. Urea, another waste product, often sits beside it and helps tell the difference between drinking too little and a real kidney problem.

The normal ranges: men 61 to 113 µmol/L from 19 to 50, up to 120 at 65, up to 134 at 80 and up to 149 above that; women 48 to 91 from 19 to 50, up to 99 at 65, up to 113 at 80 and up to 132 above that. They come from a Dutch study of more than seven million results.

Say your creatinine is 118 as a man or 96 as a woman: just above the limit. First look at your kidney function value, the eGFR. If that is 85 or higher, there is almost certainly nothing wrong with your kidneys. Then look at the days before the draw, because five ordinary things push creatinine up temporarily.

One: meat. Roasted or fried meat contains ready-made creatinine, and a large portion the evening before is still measurable the next morning. Two: drinking too little. When you are dehydrated there is less water in your blood and everything in it looks higher, creatinine included; urea rises with it. Three: exercise. A hard workout breaks down muscle fibres and that releases extra creatinine, for up to a few days. Four: creatine as a supplement. More creatine in your muscles means more creatinine in your blood, usually ten to thirty µmol/L extra, without your kidneys noticing anything. Five: an antibiotic. Trimethoprim, also in co-trimoxazole, slows the excretion of creatinine and makes the number rise ten to fifteen percent for as long as you take the course.

If none of those five applies and the value stays raised on a repeat, your kidneys really are filtering somewhat less. That almost always happens slowly and almost always has a treatable cause: high blood pressure and diabetes are the two biggest. That is why creatinine is checked every year in those conditions. A mildly raised value is then not an alarm but a reason to get blood pressure and blood sugar well controlled.

What this number does not do: detect or rule out kidney disease on its own. That is what the eGFR, a urine test for protein and the trend over several years are for. One result is a snapshot.

Creatinine is in almost every general blood test, so you often already have the value. Having it measured deliberately makes sense if you have high blood pressure or diabetes, if you take medicines that burden the kidneys, or simply once as a baseline so that you have something to compare with later.

Drink normally in the hours before the draw, skip heavy exercise the day before and do not eat a large portion of meat the evening before. If you use creatine, say so when the result is assessed. Fasting is not needed for creatinine.

A mildly raised value without symptoms you repeat after two to four quiet weeks, with eGFR alongside. If it is normal again then, it was a snapshot. If it stays raised, or your eGFR is below 60, discuss the result with your GP.