Creatinine

Your muscles store energy in creatine, and every day about two percent of it breaks down into creatinine, a waste product your body uses for nothing. The amount released is fairly constant and depends mainly on how much muscle you have: a muscular man makes more of it than a slight woman, and an older person with little muscle makes little. Your kidneys filter it out of your blood and it leaves your body with the urine. As long as production and removal are in balance, the value in your blood stays stable. That is why creatinine is the most common measure of your kidney function: if the kidneys filter less, it builds up. But the value only rises noticeably once the kidneys have already lost a good part of their capacity, and it says something only if you know how much muscle sits behind it. The same value is normal in a bodybuilder and a sign of kidneys doing less in a slight eighty-year-old. That is why the eGFR is calculated alongside, which converts creatinine with age and sex into an estimate of filtering capacity. Your report shows creatinine in µmol/L. Normal for a man of 19 to 80 years is about 64 to 104 µmol/L and for a woman from age 19 50 to 90 µmol/L, where the labs overlap; above age 80 the upper bound moves up at some of the labs.

Kidney damage does not hurt. The kidneys have so much reserve that you notice nothing of losing half their capacity, and the symptoms, tiredness, itching, holding on to fluid, only come when little is left. Creatinine is the value that makes that silent decline visible, years before you feel anything, and that is the reason it is in almost every blood test. High blood pressure and a high blood sugar are the two biggest damagers of the kidneys, and anyone with either follows their creatinine and eGFR every year. For anyone who does sport, creatinine is interesting for another reason: it says something about your muscle mass. Anyone building muscle sees the value rise slowly over months, and anyone using creatine sees it rise by ten to twenty percent without the kidneys noticing anything, because creatine is the raw material creatinine arises from. That is not a kidney problem, but it makes the eGFR too low in athletes, and it is the reason an athlete reads their creatinine with their muscle mass and their supplements alongside. The other side is just as important. In anyone with little muscle, older people, people who have been ill for long, anyone missing a leg, creatinine sits low while the kidneys are already doing less, and a normal value is no reassurance there. For them cystatin C, a second measure of kidney function that does not come from the muscles, is the better addition.

A low creatinine usually means little muscle mass, and that is rarely a problem in itself. It occurs in slight people, older people and after long illness, and in pregnancy, in which the kidneys filter harder and the blood becomes diluted. Rare are hyperfiltration, in which the kidneys temporarily work too hard early in diabetes, and a severe liver disease, in which the liver no longer makes the raw material creatine. The question with a low creatinine is not whether something is wrong, but whether the eGFR beside it presents the kidney function as better than it is. A high creatinine means your kidneys filter less, or you make more than average. The most common explanation first: reduced kidney function, and then the eGFR belongs with it and the question whether it has been so for three months; plenty of muscle mass; and dehydration, which lifts the value temporarily and recovers with drinking. After that come creatine as a supplement, a hard training session in the days before, medicines that slow excretion or lower filtration, such as some antibiotics, acid-reducing medicines, anti-inflammatory painkillers and blood pressure medicines, and high blood pressure or diabetes as the damagers in the long term. A value that rises on repeat counts more than one high value. Read creatinine beside the eGFR, which converts the number into filtering capacity, beside potassium and sodium, which show whether the kidneys are still doing their job, and with doubt about your muscle mass beside cystatin C. Protein in the urine often precedes a rise in creatinine by years.