Starting from 2015, the International Renal Interest Society (www.iris-kidney.com) includes SDMA to stage patients with CKD. The SDMA, derivative of methylation of L-arginine, is released into the bloodstream and then eliminated by renal filtration. The increase in plasma concentration of SDMA seems capable to identify, earlier than plasma creatinine, a renal function impairment. The SDMA can then be used, together with the creatinine and urea levels and the urinalysis, to identify a patient with kidney disease. Unlike plasma creatinine, SDMA is not affected by the patient's muscle mass and provides a more accurate assessment of renal function in dogs and cats with a low body condition score. A reduction of muscle mass, commonly found in older animals and in patients with CKD in advanced stages, can decrease plasma creatinine levels then leading to an incorrect staging of the nephropatic patient.