Update (June 24, 2026): IRIS CKD Staging and Substaging — Framework, SDMA Integration, and Proteinuria/BP Substages

Bottom line.

• The International Renal Interest Society (IRIS) CKD staging system classifies cats (and dogs) with stable CKD into Stages 1–4 based on fasting serum creatinine concentration and/or blood SDMA, then substages each patient for proteinuria (UP/C) and systemic arterial blood pressure.¹
• SDMA has been incorporated into the IRIS staging framework because published evidence shows blood SDMA increases earlier than creatinine in cats and dogs with progressive kidney disease, detecting GFR reductions as small as 20%.¹
• Substaging on proteinuria uses the urine protein-to-creatinine ratio (UP/C): cats with UP/C < 0.2 are non-proteinuric; UP/C 0.2–0.4 borderline proteinuric; UP/C > 0.4 proteinuric — each substage has specific management implications.¹
• Blood pressure substaging identifies cats at risk of hypertensive target-organ damage (retina, heart, brain, kidney) requiring specific antihypertensive intervention; hypertensive cats have persistent SBP 160–179 mmHg measured over 1–2 weeks; severely hypertensive ≥ 180 mmHg.¹
• This is a clinician-facing evidence summary. Always consult the most recently updated version of the IRIS staging system at www.iris-kidney.com for current thresholds.

Framework overview

• Staging basis: Fasting serum creatinine and/or blood SDMA on ≥ 2 occasions in a stable, hydrated patient.¹
• Four stages: Stage 1 (non-azotaemic with other evidence of CKD) through Stage 4 (severe azotaemia); specific creatinine and SDMA thresholds are published at the IRIS website and revised as evidence accumulates.¹
• Substaging dimensions: (1) Proteinuria by UP/C; (2) systemic arterial blood pressure.¹
• Stability requirement: Staging is only appropriate in patients with stable CKD; changes < 20–25% in creatinine/SDMA over 2–4 weeks confirm relative stability before staging.¹

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What the evidence shows

IRIS staging history and rationale

The IRIS was formed in 1998, with an early aim of developing a CKD staging system for dogs and cats to facilitate communication and guide management across the veterinary community.¹ The staging system was devised by IRIS Board members — currently 14 members from 10 countries — refined through use, and further modified in light of feedback from the American and European Societies of Veterinary Nephrology and Urology. The key principle is that a large proportion of kidney tissue must be damaged before serum creatinine rises detectably, meaning that creatinine-based staging alone may miss early disease.

Incorporating SDMA into staging

With accumulating evidence and experience of SDMA as a surrogate marker of glomerular filtration rate, the IRIS Board incorporated SDMA into the staging system.¹ SDMA concentrations increase earlier than creatinine in cats and dogs with progressive kidney disease, and SDMA is not affected by lean muscle mass — an important advantage in cats, which can have significant muscle wasting at advanced CKD stages. Persistently elevated blood SDMA above 14 μg/dL (the current upper reference limit) is consistent with a diagnosis of CKD, even in the absence of azotaemia; cats with SDMA 15–17 μg/dL would be placed in IRIS CKD Stage 1. IRIS notes that discordance between SDMA and creatinine occurs 9–17% of the time in cats, and recommends that when discordance persists the clinician should apply the higher stage as a precautionary approach.¹

Substaging on proteinuria

Substaging for proteinuria is recommended because proteinuria is a well-established independent risk factor for progressive renal injury and shorter survival in cats with CKD.¹ UP/C should be measured in all cats with CKD, provided there is no evidence of urinary tract inflammation or haemorrhage and dysproteinaemia has been excluded. For cats: UP/C < 0.2 = non-proteinuric; UP/C 0.2–0.4 = borderline proteinuric; UP/C > 0.4 = proteinuric. Persistence of proteinuria ideally requires three or more urine samples collected over at least two weeks before the substage classification triggers treatment escalation.¹

Substaging on blood pressure

Blood pressure substaging identifies cats at risk of target-organ damage to the retina (hyphema, hypertensive retinopathy), heart (left ventricular hypertrophy), brain (dullness, lethargy, seizures) and kidney.¹ Hypertensive cats have persistent systolic blood pressure 160–179 mmHg measured over 1–2 weeks; severely hypertensive cats have SBP ≥ 180 mmHg. If extra-renal target-organ damage is already present, demonstration of persistence is not necessary and treatment should begin immediately. The IRIS staging system uses a suffix "T" after the substage designator to indicate that the current level reflects treatment.

Prognostic validation of IRIS staging

The clinical utility of IRIS staging is backed by published survival data. A study of naturally occurring feline CKD diagnosed between 2000 and 2002 found that IRIS stage at the time of diagnosis was strongly predictive of survival: median survival was 1,151 days for IRIS Stage IIb cats, 778 days for Stage III, and 103 days for Stage IV, demonstrating a clear progression-associated mortality gradient that validates the staging framework in a real-world population.²

How this fits clinical practice

IRIS staging provides the organisational framework within which all feline CKD therapeutic decisions — renal diet, antiproteinuric agents, antihypertensives, phosphate binders, supportive care — are made and communicated.¹ Consistent staging at each visit enables longitudinal tracking of CKD progression, informs when to escalate treatment, and provides prognostic information for owners. Use fasting samples, ensure stability before assigning a stage, and always use the most recently updated IRIS thresholds from www.iris-kidney.com, as these are revised as new evidence accumulates. Do not infer specific management targets from this summary alone.

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References

1. Elliott J, White J. IRIS Staging System. International Renal Interest Society. 2025 (updated). https://www.iris-kidney.com/iris-staging-system
2. Boyd LM, Langston C, Thompson K, Littman MP, Valli VE. 2008. Survival in Cats with Naturally Occurring Chronic Kidney Disease (2000–2002). J Vet Intern Med 22(5):1111-1117. https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1939-1676.2008.0163.x

Changelog

• 2026-06-24: First published.