IRIS Staging in Cats: What Each CKD Stage Means (With Numbers)
Your vet just said your cat has “Stage 2 kidney disease” or “Stage 3 CKD,” and handed you a lab report full of numbers. The first question almost every owner has is the same one: what do these stages actually mean? The system your vet is using is called IRIS staging, and understanding how it works will change how you read every blood test from here on.
This guide explains what IRIS is, the exact creatinine and SDMA numbers behind each stage, the blood pressure and protein “substages” that matter as much as the stage itself, and what your cat's stage does — and does not — tell you about prognosis.
IRIS Staging in Cats: The Short Answer
IRIS staging is the international standard vets use to classify chronic kidney disease (CKD) in cats into four stages based on blood waste levels: Stage 1 (creatinine under 1.6 mg/dL with other evidence of kidney damage), Stage 2 (1.6 to 2.8 mg/dL), Stage 3 (2.9 to 5.0 mg/dL), and Stage 4 (above 5.0 mg/dL). A second marker called SDMA (under 18, 18 to 25, 26 to 38, and above 38 μg/dL respectively) is used alongside creatinine, and when the two disagree, the higher stage applies. (Our dedicated guide to SDMA in cats explains why this marker rises years before creatinine and what to do about it.) IRIS staging in cats is descriptive, not predictive — it tells your vet how much filtering capacity is left today, while the speed of change between visits tells you far more about the future than the stage number alone.
What Is IRIS?
IRIS stands for the International Renal Interest Society, an independent group of veterinary nephrology specialists founded in the late 1990s. Before IRIS, vets described kidney disease with loose terms like “mild renal failure” or “chronic renal insufficiency” — phrases that meant different things in different clinics and made research almost impossible to compare. IRIS replaced that with a numbered staging system that any vet, anywhere, can apply to the same lab values and reach the same conclusion.
Two things are worth knowing about how IRIS works. First, it is a guideline, not a regulator — vets voluntarily adopt it, and it is revised as the evidence changes (most recently modified in 2023, which formally incorporated SDMA into staging). Second, IRIS staging is only applied to chronic kidney disease: the kidney values must be persistently abnormal, typically for at least three months, before a cat is staged at all. A single bad number during an illness or dehydration episode is not CKD.
The IRIS CKD Staging Table for Cats
Here is the full IRIS staging table for cats, using the blood markers your vet measures on a standard chemistry panel. Creatinine is the classic waste marker; SDMA is a newer marker that rises earlier in kidney disease.
| IRIS Stage | Creatinine (mg/dL) | Creatinine (µmol/L) | SDMA (µg/dL) | What It Means |
|---|---|---|---|---|
| Stage 1 | Under 1.6 | Under 140 | Under 18 | Kidney damage present, but waste levels still normal |
| Stage 2 | 1.6 – 2.8 | 140 – 250 | 18 – 25 | Mild waste buildup; usually the first stage found on bloodwork |
| Stage 3 | 2.9 – 5.0 | 251 – 440 | 26 – 38 | Moderate CKD; symptoms usually visible |
| Stage 4 | Above 5.0 | Above 440 | Above 38 | End-stage; care focuses on comfort and quality of life |
Note that these cutoffs are for cats. Dogs are staged with slightly different numbers (their Stage 2 starts at a lower creatinine), so if you find a staging chart online that does not match the one above, check which species it was written for.
How Vets Actually Assign the Stage
IRIS rules require that creatinine and SDMA be assessed on at least two separate occasions — ideally two or more weeks apart — in a cat who is hydrated, stable, and not fasting abnormally, before a stage is assigned. This is why your vet may say “let's recheck in two to four weeks” instead of giving you a stage on the spot. A cat who is dehydrated from vomiting, or who has another illness raging, will have falsely high kidney values that settle once they are treated.
When the creatinine-based stage and the SDMA-based stage disagree, IRIS says to use the higher of the two. So a cat with creatinine of 1.9 mg/dL (Stage 2) but SDMA of 30 µg/dL (Stage 3) is staged as Stage 3. One more caveat: creatinine comes from muscle, so a very thin, muscle-wasted cat can have a deceptively “better” creatinine than their kidneys deserve — another reason SDMA, which is less affected by muscle mass, is measured alongside it.
IRIS SDMA: Why Two Markers?
SDMA (symmetric dimethylarginine) was added to IRIS staging because creatinine has a blind spot. Cats can lose up to roughly 70 percent of kidney function before creatinine leaves the normal range — the kidneys have enormous reserve, and creatinine only climbs once that reserve is exhausted. SDMA rises earlier, typically when about 40 percent of function is lost, which is why so many cats are now diagnosed at Stage 1 or early Stage 2 instead of Stage 3. If your cat's SDMA is elevated but creatinine is still normal, you are catching the disease years earlier than the previous generation of owners could — and early is exactly when diet and monitoring have the most leverage.
The Substages: Blood Pressure and Proteinuria
The stage number is only half of the IRIS label. Every staged cat also gets two substages — one for blood pressure and one for protein in the urine — because both can quietly accelerate kidney damage and both change what your vet prescribes.
| Blood Pressure Substage | Systolic BP (mmHg) |
|---|---|
| Normotensive | Under 140 |
| Prehypertensive | 140 – 159 |
| Hypertensive | 160 – 179 |
| Severely hypertensive | 180 or higher |
Most veterinary internists start antihypertensive medication (typically amlodipine or telmisartan for cats) once systolic pressure is persistently at or above 160 mmHg, with the aim of bringing it into the 120 to 150 range. Uncontrolled hypertension does not just damage kidneys — it also detaches retinas and causes sudden blindness, which is why the blood pressure reading is checked at every staging visit.
The second substage uses the urine protein:creatinine ratio (UPC). In cats, a UPC under 0.2 is non-proteinuric, 0.2 to 0.4 is borderline, and above 0.4 is proteinuric. Persistent proteinuria at any stage warrants a closer look — urine culture, blood pressure recheck, and sometimes medication — because protein leaking through the kidney filters is both a marker of damage and a driver of it.
Is the Stage a Life Sentence?
No — and this is the most important paragraph on this page. IRIS staging describes how much filtering capacity is left today. It is not a countdown. Published studies of cats diagnosed at Stage 2 report median survival measured in years — many cats live two to three years or more after a Stage 2 diagnosis, and some stay stable for far longer. Even at Stage 3, median survival in studies is commonly reported in the range of one to two years, and with good symptom control (nausea management, hydration, phosphorus control) many cats do better than the median. Stage 4 is genuinely end-stage, and care there shifts toward comfort — but even then, weeks to months of good-quality time is a realistic goal.
What predicts the future better than the stage itself is the rate of progression: how fast creatinine and SDMA climb between visits. A Stage 3 cat whose numbers are stable for a year has a completely different outlook from a Stage 2 cat whose creatinine doubled in three months. This is why your vet keeps a trend line rather than a single value — and why we recommend keeping your own copy of every blood panel. If you want to go deeper on the single most important number, our guide to understanding your cat's creatinine levels breaks down what each range means and what moves it.
Can a Cat's IRIS Stage Go Down?
Yes, sometimes. A cat staged during a dehydrated crisis — a vomiting episode, a hot week with poor water intake, or an acute-on-chronic injury — may genuinely restage one or even two stages lower once rehydrated and treated. What does not happen is true reversal: lost nephrons do not grow back, so the kidneys themselves never return to an earlier stage. The practical takeaway is that the first stage assigned during an illness is provisional. Restage after your cat is stable before making any big decisions, and never let a single scary number drive end-of-life choices.
How Often Should a CKD Cat Be Restaged?
- Stage 1: every 3 to 6 months — the goal is catching the trend early, not treating yet
- Stage 2: every 3 months once values are stable, and whenever symptoms change
- Stage 3: every 2 to 3 months, because medication doses and phosphorus targets shift with the numbers
- Stage 4: monthly or as symptoms dictate; comfort becomes the primary metric
A restaging visit is more than bloodwork: it typically includes creatinine, SDMA, phosphorus, potassium, packed cell volume, urine specific gravity, UPC, blood pressure, and a weigh-in. You can estimate your cat's stage between visits with our free IRIS stage calculator.
Find Your Cat's Stage
Each stage has its own playbook — what to monitor, what to feed, which medications typically enter the picture, and what quality of life looks like. Go to the guide for your cat's stage: