The IRIS Staging System for CKD in Cats - What Every Cat Parent Needs to Understand
If your cat has been diagnosed with CKD, your vet will use the IRIS staging system to guide every treatment decision. Here's what each stage means - in plain language.
When a cat is diagnosed with Chronic Kidney Disease, the conversation with your vet will almost certainly include a number - Stage 1, Stage 2, Stage 3, or Stage 4. This number comes from a system developed by the International Renal Interest Society, known as IRIS - an independent body of veterinary specialists whose focus is kidney disease in cats and dogs.
Understanding your cat's IRIS stage is not optional information. It is the framework that determines what your cat should eat, what medications are appropriate, how frequently they need monitoring, and what signs to watch for at home. Every management decision in CKD is stage-dependent.
This post explains the IRIS staging system clearly, so you can have more informed conversations with your vet and make better decisions for your cat.
What IRIS Staging Is Based On
IRIS staging is primarily based on a blood marker called creatinine - a waste product that healthy kidneys filter out of the body continuously. When kidney function declines, creatinine accumulates in the bloodstream. The higher the creatinine level, the more advanced the kidney disease.
A second marker called SDMA - symmetric dimethylarginine - is increasingly used alongside creatinine. SDMA is more sensitive and can detect kidney decline earlier, sometimes months before creatinine rises into abnormal ranges. Many vets now include SDMA in routine senior blood panels for this reason.
Both values are measured through a standard blood test. If your cat has been diagnosed with CKD, these numbers will appear on every blood report going forward.
The Four IRIS Stages
At Stage 1, kidney function is measurably reduced - but the body is still compensating well enough that symptoms are absent. Most cats at this stage look and behave completely normally. This is why Stage 1 is almost never caught without routine blood work. Management at this stage is largely preventive - diet optimisation, hydration support, and monitoring every 3 to 6 months.
Stage 2 is where most cats are when CKD is first diagnosed. Subtle signs may begin to appear - slightly increased thirst, more frequent urination, occasional reduced appetite. At Stage 2, dietary intervention becomes non-negotiable. A renal prescription diet - low in phosphorus, controlled in protein - is typically introduced. Monitoring increases to every 3 months.
By Stage 3, symptoms are more visible - weight loss, lethargy, reduced appetite, occasional vomiting. Management becomes more intensive: renal prescription diets are essential, phosphorus binders may be introduced, and subcutaneous fluid therapy often begins. Blood pressure medication may be required. Monitoring moves to every 1 to 3 months.
Stage 4 represents significant kidney failure. The cat requires active, ongoing medical management and close veterinary supervision. Quality of life becomes the central consideration alongside treatment decisions. Management at this stage is highly individualised and determined in close partnership with your vet.
Beyond the Stage Number - The Two Substages
IRIS staging does not stop at creatinine. Every stage is further classified by two additional markers that meaningfully change the management plan.
Hypertension is extremely common in CKD cats and causes its own damage - to the kidneys, eyes, and brain. IRIS classifies blood pressure risk from minimal to severe. A cat at Stage 2 with severe hypertension requires very different management from a Stage 2 cat with normal blood pressure.
When kidneys are damaged, protein leaks into the urine. This is measured by the UPC ratio. Elevated proteinuria accelerates CKD progression and typically requires additional medication. IRIS classifies proteinuria as non-proteinuric, borderline, or proteinuric.
Two cats at the same creatinine level can have very different prognoses and management plans, depending on their blood pressure classification and UPC ratio. The stage number gives you context. The substages give you the complete picture.
How to Use This Information
Understanding IRIS staging gives you three practical tools as a cat parent.
First, it gives you a shared language with your vet. When your vet says "she's stable at Stage 2," you now know what that means - and what questions to ask about blood pressure and UPC ratio.
Second, it gives you a benchmark for monitoring. When you retest in three months, you are not just looking at whether the numbers are high or low. You are asking a specific question: has the stage held, or has it progressed? Stability is a success at any stage.
Third, it gives you direction for every management decision that follows - diet, hydration, environment, medications. In this CKD series, every recommendation we make will reference the IRIS stage it applies to. This post is your reference point for everything that follows.
A Note on SDMA and Early Detection
One of the most important developments in feline CKD management in recent years is the wider availability of SDMA testing. Because SDMA rises earlier than creatinine, it can identify kidney decline at a point where intervention has the most impact.
If your cat is over seven years old and has not had an SDMA test, it is worth asking your vet to include it in the next blood panel. CKD caught at Stage 1 gives you the most time and the most options.
Annual blood work for senior cats is not overcaution. For kidney disease specifically, it is the difference between catching a problem early and managing a crisis.
This post is part of Gishiki Guides - CKD, a series on feline Chronic Kidney Disease written for cat parents navigating a diagnosis. All content is for educational purposes only. Always consult your veterinarian before making any changes to your cat's diet, medication, or care plan.
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