IRIS staging for CKD: stages 1 to 4 explained

Articles · Kidney Disease

If your cat has been diagnosed with chronic kidney disease (CKD), your vet has probably mentioned an “IRIS stage.” This staging system is the common language vets use to describe how advanced kidney disease is and to guide monitoring and treatment. This guide explains what IRIS staging is, what each of the four stages means, and what questions are worth asking at your cat’s next appointment.

What is IRIS staging?

IRIS staging is a system created by the International Renal Interest Society to classify chronic kidney disease in cats and dogs into four stages, mainly based on blood creatinine and SDMA levels in a stable, well-hydrated patient. It gives vets a consistent way to describe disease severity and to match monitoring and treatment to the stage.

The system was designed to standardize CKD care worldwide. Rather than every clinic using its own informal labels, IRIS provides clear cutoffs so a “stage 2” cat means the same thing everywhere. Crucially, staging is meant to be done when a cat is stable and properly hydrated — not in the middle of a dehydration crisis — because temporary factors can push creatinine up. According to IRIS, staging is based on fasting blood creatinine and/or SDMA, then further refined by two “substages”: proteinuria and blood pressure. We will come back to those substages, because they matter as much as the number.

How are the four IRIS stages defined?

The four stages are defined by progressively higher creatinine and SDMA values. Per the IRIS 2023 guidelines, the feline cutoffs are roughly: Stage 1 — creatinine under 1.6 mg/dL; Stage 2 — 1.6 to 2.8 mg/dL; Stage 3 — 2.9 to 5.0 mg/dL; Stage 4 — over 5.0 mg/dL. SDMA bands run alongside these.

Here is the fuller picture from the IRIS 2023 staging guidelines, using creatinine (mg/dL) and SDMA (µg/dL):

• Stage 1: Creatinine under 1.6; SDMA under 18. Kidney damage is present but creatinine is still normal. Often found incidentally; cats usually show no signs.
• Stage 2: Creatinine 1.6 to 2.8; SDMA 18 to 25. Mild azotemia. Many cats still look healthy, though some early signs like increased drinking may appear.
• Stage 3: Creatinine 2.9 to 5.0; SDMA 26 to 38. Moderate disease. Clinical signs become more common and treatment intensifies.
• Stage 4: Creatinine over 5.0; SDMA over 38. Advanced disease with a high risk of uremic signs.

IRIS notes that when SDMA and creatinine point to different stages, the higher stage generally guides treatment — for example, a cat with stage 2 creatinine but persistently high SDMA may be managed as stage 3. Our SDMA explainer covers why this marker is so useful.

Why do SDMA and creatinine both appear in staging?

Both markers appear because they capture kidney function in complementary ways, and using them together catches disease earlier and more reliably. Creatinine is the long-standing standard, but it is affected by muscle mass and rises late. SDMA tends to rise earlier and is not skewed by a poorly-muscled senior cat.

According to the Cornell Feline Health Center, creatinine concentrations generally do not rise until a cat has lost almost 75% of kidney function, whereas SDMA elevations can be detectable when about 40% of function is lost. That earlier signal is why IRIS incorporated SDMA into staging. Because SDMA is not affected by muscle mass, it is often more reliable in a thin, elderly cat whose creatinine might otherwise look falsely reassuring. Reading the two values together — ideally as a trend across rechecks — gives a far more accurate stage than either alone. If interpreting these numbers feels daunting, our lab value translator explains each one in plain English.

What are the substages, and why do they matter?

Substages describe two additional factors layered on top of the main stage: proteinuria (protein leaking into urine) and systemic hypertension (high blood pressure). Both are important because they independently affect how fast CKD progresses and how a cat feels, so your vet manages them separately from the stage number itself.

Proteinuria is assessed with the urine protein-to-creatinine ratio (UPC). Per IRIS, cats are classed as non-proteinuric (UPC under 0.2), borderline (0.2 to 0.4), or proteinuric (over 0.4). This matters because, according to IRIS-cited research, each 0.1 increase in UPC was associated with a roughly 24% increase in the risk of CKD progression. For blood pressure, IRIS notes that about 20% of CKD animals are hypertensive at diagnosis, with another 10-20% developing it over time, which is why all CKD cats should have blood pressure checked at diagnosis and regularly thereafter. A cat’s “full” stage really is the number plus these two substages together.

What does my cat’s stage mean for monitoring?

Your cat’s stage largely determines how often they need rechecks and how intensive treatment is. In general, earlier and stable stages are monitored less frequently — often every six months — while later or unstable stages need rechecks every one to three months. Your vet sets the exact schedule for your cat.

Higher stages also typically mean more layers of management: a renal diet, possibly phosphate binders, blood pressure or proteinuria medications, and closer attention to hydration and weight. The stage is not a countdown — staging at diagnosis is a useful prognostic guide, but many cats live well within their stage for a long time with good care. It is worth knowing that initial staging can occasionally overestimate severity if a cat was dehydrated; one study found about 30% of cats first classed as stage 4 moved to a lower stage after stabilization. This is exactly why restaging when stable matters. Tracking trends over time is central to how we think about CKD care.

What should I ask my vet about my cat’s stage?

Useful questions include: What IRIS stage is my cat, and is that based on stable, hydrated values? What are the proteinuria and blood pressure substages? What is the target for phosphorus and other key values? And how often should we recheck? These questions turn a stage into a concrete care plan.

It also helps to ask what specific signs at home should prompt an earlier visit, and whether any current values are trending in a concerning direction. Coming to the appointment with your cat’s recent lab history organized — creatinine, SDMA, phosphorus, UPC, and blood pressure over time — lets your vet answer these questions precisely instead of working from a single recent result. Our FAQ covers more of the common questions CKD cat parents ask.

To recap: IRIS staging sorts CKD into four stages using creatinine and SDMA in a stable cat, then refines the picture with proteinuria and blood pressure substages. The stage guides monitoring frequency and treatment intensity, but it is a guide, not a verdict — and it should always be set when your cat is stable.

Pawtient AI’s trends view keeps every creatinine, SDMA, and phosphorus result in one timeline so the staging picture is easy to follow over time — see how Pawtient AI supports CKD parents. Pawtient AI is an AI assistant and second opinion, never a diagnosis — always consult your veterinarian.

Sources

• International Renal Interest Society (IRIS). “IRIS Staging of CKD (modified 2023).” https://www.iris-kidney.com/iris-staging-system
• Cornell Feline Health Center. “Chronic Kidney Disease.” Cornell University College of Veterinary Medicine, 2022. https://www.vet.cornell.edu/departments-centers-and-institutes/cornell-feline-health-center/health-information/feline-health-topics/chronic-kidney-disease
• IDEXX. “IRIS CKD Staging Guidelines for Cats and Dogs.” 2023. https://www.idexx.com/en/veterinary/reference-laboratories/sdma/sdma-iris/

By Pawtient AI Editorial Team. Educational content reviewed against published veterinary guidelines (IRIS, AAHA, WSAVA, ACVIM, AAFP). Not a substitute for veterinary care.