Creatinine Measurement of creatinine concentration in serum is included in chemistry profiles mainly to screen for decreased glomerular filtration rate (GFR). Concentrations of serum creatinine (SC) are expressed in mg/dl. Creatinine is produced as the result of normal muscle metabolism. Phosphocreatine, an energy-storing molecule in muscle, undergoes spontaneous cyclization to form creatine and inorganic phosphorous. Creatine then decomposes to creatinine. Production of endogenous creatinine is quite stable. An additional and relatively minor source is creatinine ingested during consumption of animal tissue and absorbed from the intestines. Creatinine is filtered freely through the glomerulus and is not reabsorbed in the tubules. Therefore, creatinine is a more reliable measure of GFR, compared to UN, in all species as it is not influenced by diet or protein catabolism. Causes of increased creatinine Artifact: When measured by the Jaffe technique (which is based on color production and is used by the chemistry analyzer at Cornell University), both creatinine and non-creatinine chromogens react with the reagent. Non-creatinine chromogens include acetoacetate, glucose, vitamin C, uric acid, pyruvate, cephalosporins and amino acids. When present in high concentrations, these can artefactually elevate creatinine values. Physiologic: Higher in foals (up to 8 mg/dL; thought to be due to defective placental transfer) and heavily muscled horses (up to 2.5 mg/dL). Decreased GFR: Due to prerenal, renal or post-renal causes. In ruminants and horses, creatinine is the best indicator of GFR. Increased production: A mild increase (< 1 mg/dL) may be seen after ingestion of a recent meat meal. Acute myositis does not consistently increase creatinine itself (although severe myositis or myopathy can produce renal azotemia from myoglobinuric nephrosis). Causes of decreased creatinine Artifact: With the Jaffe reaction, severe icterus (total bilirubin > 10 mg/dL or an icteric index > 10 units) may artefactually decrease creatinine concentrations (this is based on data in humans and may not occur in animals or in every icteric animal). This effect can be minimized by utilizing a reaction blank. Decreased production: Loss of muscle mass, young puppies (low muscle mass). Severe liver disease from cirrhosis may result in decreased creatinine values from decreased creatine production. Increased GFR: This occurs in animals with portosystemic shunts and during pregnancy (due to increased cardiac output).