When should SDMA be used instead of creatinine for staging kidney disease in dogs and cats?

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SDMA should replace creatinine as the primary staging biomarker — or be used to upstage — in four specific clinical scenarios: muscle-wasting patients, early-stage disease detection, growing dogs, and cases where creatinine and SDMA are discordant. SDMA detects kidney dysfunction earlier than creatinine in both species. SDMA increases above the reference interval at an average 40% reduction in GFR, whereas creatinine does not rise until approximately 75% of kidney function is lost.35 In dogs, SDMA increased before creatinine by a mean of 9.8 months (range 2.2–27.0 months).2 In cats, SDMA detected CKD an average of 17.0 months before creatinine exceeded the reference interval.2 For this reason, a persistently elevated SDMA with a normal creatinine is not a false positive to dismiss — it identifies animals at meaningful risk of progressive disease.13 In patients with reduced lean body mass, creatinine alone will underestimate the IRIS stage and SDMA should take precedence. Creatinine is produced by muscle, so cachexia or muscle wasting falsely lowers creatinine and can mask significant GFR loss.16 SDMA is unaffected by lean body mass and provides a more accurate reflection of GFR in these patients.146 This is particularly relevant in cats with concurrent illness causing muscle loss, where creatinine-only staging has been shown to underestimate the true IRIS CKD stage.6 In growing dogs, SDMA is preferred over creatinine for tracking CKD progression. Creatinine rises physiologically during growth, which can overestimate the rate of CKD progression when inverse biomarker slopes are used to distinguish stable from progressive disease. Because SDMA is not affected by growth, it more accurately reflects the onset of declining GFR in young dogs with hereditary nephropathies.14 When SDMA and creatinine are discordant, the combination outperforms either marker alone. In cats and dogs with persistently increased creatinine, 70–75% had a concordantly elevated SDMA at the same ti