Creatinine is a breakdown product of muscle creatine phosphate. The daily amount of creatinine released from the muscles depends mainly on muscle mass. After release from muscles, creatinine is carried to the kidneys, where almost 100% of it is excreted in urine. Creatinine retention can be caused by reduced renal filtration, lower urinary tract disorders or impaired renal blood flow.
Blood creatinine is commonly used as a measure of kidney function. International recommendations advise, that in the early stages of renal disease, creatinine concentrations should be compared to the previous result of the same dog instead of the reference ranges. This approach enables detection of slight elevations in the creatinine concentration, that would otherwise go unnoticed. When using this kind of approach, only results obtained by the same analyzer should be compared. For example, results obtained by PetMETA can be lower than results obtained using a routine clinical chemistry analyzer. This is also the reason for using only analyzer-specific reference ranges. Creatinine should be measured in fasting samples, as feeding can increase blood creatinine levels.