What is the IRIS staging system?

The International Renal Interest Society (IRIS) system is the global standard for classifying chronic kidney disease in dogs and cats. It stages CKD 1–4 using fasting blood creatinine and SDMA, then substages by urine protein-to-creatinine ratio (UPC) and systolic blood pressure. It is for confirmed, stable CKD — not for diagnosing kidney disease or grading acute kidney injury.

What are the creatinine and SDMA thresholds for each stage?

Stage 1: creatinine below 1.4 mg/dL (dog) / 1.6 mg/dL (cat), SDMA below 18 µg/dL. Stage 2: creatinine 1.4–2.8 (dog) / 1.6–2.8 (cat), SDMA 18–35 (dog) / 18–25 (cat). Stage 3: creatinine 2.9–5.0 both species, SDMA 36–54 (dog) / 26–38 (cat). Stage 4: creatinine above 5.0, SDMA above 54 (dog) / 38 (cat).

When should IRIS staging be applied?

Only in a confirmed, stable, well-hydrated patient. Acute kidney injury, dehydration, and post-renal obstruction must be ruled out or resolved first, because a dehydrated animal can look artificially azotemic. Staging on unstable values produces a stage that misleads both prognosis and treatment.

What if creatinine and SDMA give different stages?

Consider the patient's muscle mass, then repeat both after about 14 days. If the discrepancy persists, use the higher stage (usually the SDMA-derived one), because SDMA rises earlier and is muscle-mass independent. This is the most common source of staging confusion in thin, elderly patients.

Two substages refine the prognosis: proteinuria (UPC) and systemic blood pressure. Non-proteinuric is UPC below 0.2; proteinuric is above 0.5 (dog) / 0.4 (cat). Hypertension substaging uses systolic blood pressure with organ-damage risk. A stage 2 dog that is also proteinuric and hypertensive is managed far more aggressively than one that is not.

IRIS CKD Staging in Dogs and Cats: The 4-Stage System

Q: Why does stage 1 need extra evidence?

Because stage 1 creatinine and SDMA are normal or only mildly elevated, so they do not diagnose CKD on their own. A stage 1 diagnosis requires other persistent abnormalities — raised SDMA (15–17 µg/dL), renal proteinuria, poor urine concentrating ability, or abnormal renal imaging — to confirm that kidney disease is actually present.

TL;DR — The IRIS system is how the world talks about chronic kidney disease in dogs and cats. It stages CKD 1–4 using fasting creatinine and SDMA — stage 1 is non-azotemic (creatinine <1.4 dog / <1.6 cat mg/dL), stage 4 is end-stage (creatinine >5.0) — then substages by proteinuria (UPC) and blood pressure. Two rules keep it honest: (1) stage only a confirmed, stable, hydrated patient — a dehydrated animal looks falsely azotemic; and (2) stage 1 does not diagnose CKD on its own, because the numbers are normal — it needs a persistently elevated SDMA (15–17 µg/dL), proteinuria, or poor urine concentration to confirm disease. The stage is a shared language, not a death sentence; the substage is what actually drives treatment.

In Plain Terms

When two vets talk about a “stage 2 cat,” they need to mean the same thing. That is what IRIS provides — a common vocabulary for how far kidney disease has progressed, so prognosis, treatment and referral all line up. Think of the stage as the address (where the disease is), and the substage as the details (how aggressive to be about it).

The Four Stages, Exactly

Stage	Dog creatinine	Cat creatinine	Dog SDMA	Cat SDMA
1	< 1.4 mg/dL	< 1.6 mg/dL	< 18 µg/dL	< 18 µg/dL
2	1.4–2.8 mg/dL	1.6–2.8 mg/dL	18–35 µg/dL	18–25 µg/dL
3	2.9–5.0 mg/dL	2.9–5.0 mg/dL	36–54 µg/dL	26–38 µg/dL
4	> 5.0 mg/dL	> 5.0 mg/dL	> 54 µg/dL	> 38 µg/dL

A critical nuance for stage 1: the upper limit of the SDMA reference interval is 14 µg/dL, and a persistent 15–17 µg/dL places a patient in stage 1 — even when creatinine is normal. This is the mechanism by which SDMA catches CKD before creatinine moves (SDMA rises at ~40% GFR loss vs ~75% for creatinine).

The Two Rules That Prevent Misuse

Rule 1 — Stage only a stable, hydrated patient

IRIS staging is not for diagnosis and not for acute kidney injury. A dehydrated dog or one with post-renal obstruction can look artificially azotemic — stage that patient and you will over-stage the disease, over-promise a poor prognosis, and over-treat. AKI, dehydration, and obstruction must be ruled out or resolved first, and values should be measured on two occasions in a hydrated, stable patient.

Rule 2 — Stage 1 needs supporting evidence

Stage 1 creatinine and SDMA are normal or near-normal, so the stage does not, by itself, prove kidney disease. A stage 1 label requires another persistent abnormality: elevated SDMA (15–17 µg/dL), renal proteinuria, poor urine concentration, or abnormal renal imaging. This is why a “normal” chemistry panel is not a clean bill of kidney health in an at-risk animal.

When Creatinine and SDMA Disagree

This is the most common staging headache, and it happens most often in thin, elderly cats. The sequence:

Consider muscle mass — a cachectic patient can have a falsely low creatinine.
Repeat both markers after ~14 days.
If the discrepancy persists, use the higher stage — which is usually the SDMA-derived one, because SDMA is muscle-mass independent and rises earlier.

The practical upshot: when creatinine says stage 1 and SDMA says stage 2, trust the SDMA and recheck — you are likely looking at early CKD that creatinine has not caught up to yet.

Substaging: What Actually Drives Treatment

The stage alone is incomplete. Two substages turn it into an action plan:

Proteinuria (UPC)

Substage	Dog UPC	Cat UPC
Non-proteinuric	< 0.2	< 0.2
Borderline	0.2–0.5	0.2–0.4
Proteinuric	> 0.5	> 0.4

Proteinuria is an independent driver of progression — in dogs, a UPC above 1.0 meant ~3× the risk of uremic crisis/death. A proteinuric stage 2 patient is managed more aggressively (earlier ACE inhibitor, tighter phosphorus control) than a non-proteinuric one at the same stage.

Blood pressure

Hypertension causes target-organ damage (retina, kidney, brain, heart), so systolic blood pressure is classified by organ-damage risk and managed with the same seriousness as the azotemia itself. The two substages together — proteinuria and hypertension — explain most of the difference in how two “stage 2” patients are actually treated.

Putting It Together: A Worked Example

A 12-year-old cat presents with polyuria/polydipsia. Fasting bloodwork: creatinine 2.1 mg/dL, SDMA 22 µg/dL, UPC 0.6, blood pressure 175 mmHg.

Stage: creatinine 2.1 → stage 2; SDMA 22 → stage 2 (agree).
Proteinuria substage: UPC 0.6 > 0.4 → proteinuric.
Blood pressure substage: 175 mmHg → hypertensive with organ-damage risk.

Result: a stage 2, proteinuric, hypertensive cat — managed with a renal diet, phosphorus control, an ACE inhibitor for the proteinuria, and antihypertensive therapy. A cat with the same creatinine but a UPC of 0.1 and normal blood pressure would be stage 2 non-proteinuric, normotensive — and treated far more gently. Same stage, very different plans.

How the Test Runs

Confirm CKD is the right framework (stable, hydrated, AKI and obstruction ruled out).
Measure fasting creatinine and SDMA on two occasions.
Stage 1–4 from the table.
Substage by UPC and systolic blood pressure.
Re-stage periodically to track progression and adjust therapy.

Related products: SDMA Chronic Kidney Disease Kit · FIA680 Analyzer · FIA880 Analyzer

FAQ

What is IRIS staging?

4 stages, 2 substages — the global standard for CKD: stage 1–4 by creatinine and SDMA, then substage by UPC and blood pressure. For confirmed, stable CKD only.

What are the exact thresholds?

Stage 1: creatinine <1.4 (dog) / <1.6 (cat) mg/dL, SDMA <18 µg/dL. Stage 4: creatinine >5.0, SDMA >54 (dog) / >38 (cat). Full table above.

When should I stage?

Only a stable, hydrated patient measured on 2 occasions — AKI, dehydration and obstruction must be ruled out first, or you’ll over-stage an artificially azotemic animal.

What if creatinine and SDMA disagree?

Check muscle mass, repeat in 14 days, then use the higher stage (usually SDMA’s) — a thin patient’s creatinine reads falsely low.

What is substaging?

UPC + blood pressure — non-proteinuric <0.2; proteinuric >0.5 (dog) / >0.4 (cat); plus systolic BP with organ-damage risk. The substage drives treatment intensity.

Why does stage 1 need extra evidence?

Because its numbers are normal — a stage 1 label requires a persistent SDMA of 15–17 µg/dL, proteinuria, or poor urine concentration to confirm disease actually exists.

Key Takeaways

Memorize the four-stage table — creatinine <1.4/1.6 (stage 1) to >5.0 (stage 4), with SDMA <18 up to >54/38 µg/dL. The stage is the shared language of CKD.
Stage only stable, hydrated patients — a dehydrated animal looks falsely azotemic. Stage on 2 occasions; stage an AKI or obstruction and you over-stage the disease.
Stage 1 doesn’t diagnose itself — normal creatinine needs a persistent SDMA of 15–17 µg/dL, proteinuria, or poor urine concentration to confirm early CKD.
When markers disagree, trust the higher — check muscle mass, repeat in 14 days, use the higher stage. A thin cat’s creatinine lies low.
The substage drives treatment — a proteinuric (>0.5 dog / >0.4 cat) and hypertensive stage 2 patient is treated far more aggressively than a non-proteinuric one at the same stage. Stage = address; substage = plan.

References

IRIS (International Renal Interest Society) staging system: https://www.iris-kidney.com/iris-staging-system
IRIS proteinuria guidelines: https://www.iris-kidney.com/proteinuria
Hall JA, et al. Comparison of SDMA, cystatin C, and creatinine for detection of decreased GFR in dogs. J Vet Intern Med. 2018.
Elliott J, Watson ADJ. IRIS staging of CKD — overview and practical application. Vet Clin North Am Small Anim Pract. 2016.

This content is for educational and product-selection purposes only. It is not a substitute for veterinary diagnosis — any animal with suspected kidney disease should be evaluated by a veterinarian. Reference ranges are platform-dependent; always use the intervals validated for your specific analyzer. Product specifications are as published by Migibio (Guangzhou Magic Biotech Co., Ltd.) and may change.