Creatinine (urine reference value)

A reference value for normalising urine biomarkers. It indicates urine concentration. Also a kidney function marker (serum creatinine).

What creatinine shows

The liver and kidneys make creatine from the amino acids arginine, glycine and methionine; creatine also comes from meat and fish. The muscles store it as creatine phosphate, a fast energy buffer that supplies ATP in the first seconds of an exertion. Every day, a small, fairly constant part of it is converted into creatinine without the involvement of an enzyme. Creatinine has no further function; the kidneys filter it freely from the blood and hardly recover it, so the daily amount excreted depends mainly on muscle mass. In a urine sample, the creatinine value therefore shows how concentrated the urine is – other urine values are related to it so that fluid intake and time of day distort the result less. In the blood, by contrast, creatinine shows how well the kidneys filter.

Low creatinine – what does it mean?

A low value in urine usually shows diluted urine – after drinking plenty or with a sample taken shortly after the last urination; the urine values related to it then become less precise. Less creatinine is also formed with low muscle mass, for example in older age, with lack of exercise or malnutrition, and with a meat-free diet, because less creatine is taken in. In the blood, low values are usually unremarkable for the same reasons; in pregnancy they fall because the kidneys receive more blood flow and filter more.

High creatinine – what does it mean?

A high value in urine shows concentrated urine – for example with low fluid intake, heavy sweating or a morning sample – or a large muscle mass. A lot of meat, creatine supplements and intense muscular work also increase the amount of creatinine. When creatinine rises in the blood, the kidneys are filtering less: along with creatinine, other substances normally excreted in urine, water and salts then remain in the body for longer. Because the blood level only rises once filtering capacity has already declined markedly, and also depends on muscle mass, the estimated filtration rate (eGFR) is calculated from it together with age and sex.

The interpretation is given in your report.