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Creatinine is formed as a degradation product of creatine kinase, an enzyme mainly found in skeletal muscle. Its rate of production is proportional to the muscle mass of the animal and very muscular animals and entire male animals often have higher serum concentrations than others. It is filtered passively into the glomerular filtrate and it is not reabsorbed so the urine creatinine concentration can be used to standardise the concentrations of other metabolites in urine, as in the calculation of the urine protein: creatinine ratio (UPC) or urine corticoid: creatinine ratio (UCCR) for hyperadrenocorticism.

Clinical Significance

Elevations in serum creatinine concentration are most likely to occur with reductions in glomerular filtration rate (GFR) as occurs in renal failure. An elevated creatinine concentration with a raised urea concentration is termed azotaemia and this occurs almost exclusively with renal failure. In animals with chronic kidney disease, more creatinine may be excreted through the gastro-intestinal tract, reducing the apparent severity of the renal disease.

Creatinine may also be elevated in animals that have undergone recent muscle catabolism or myopathies but the concentration of creatine kinase (CK) should also be elevated in these patients.

The serum creatinine concentration may be reduced in animals with a reduced muscle mass.