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A stage is a rough measure of how much kidney function your cat has already lost, on a scale of 1 to 4. The stages of kidney disease in cats are a planning tool first: they set the monitoring schedule and point your vet at the right treatment. Stage does track with survival across large groups of cats, but the spread inside every stage is wide enough that the number cannot forecast one individual.
The system comes from the International Renal Interest Society (IRIS), a group of veterinary nephrologists who built it so vets anywhere could compare notes using the same yardstick. A cat is placed in a stage using fasting blood creatinine, fasting SDMA, or preferably both, measured on at least two occasions in a hydrated, stable patient [IRIS Staging of CKD, 2026 edition]. Only after that is the stage refined into substages using urine protein and blood pressure.
Two things follow from that, and owners often miss both. Staging happens after kidney disease has already been diagnosed. A raised creatinine or SDMA on its own is not a diagnosis [IRIS Treatment Recommendations for CKD in Cats, 2026]. A dehydrated or acutely unwell cat also cannot be staged until it is stable, because dehydration alone pushes creatinine up.
Why Do Veterinarians Track Stages of Kidney Disease in Cats?
Chronic kidney disease is common in cats and usually progresses slowly, so vets need a consistent way to track it over time and adjust care as it moves.
Staging rests on fasting creatinine and fasting SDMA together. Creatinine is cheap, available everywhere and easy to trend. It also depends on muscle mass, so a thin older cat can look better on paper than its kidneys really are. SDMA is less affected by muscle loss, which is why IRIS asks for both where they are available.
Staging describes stable, established kidney disease. It needs at least two fasting samples from a hydrated, stable cat, not one bad day at the clinic. A dehydrated or acutely sick cat gets treated first and staged afterwards.
The two markers can disagree, and IRIS says what to do when they do. If creatinine sits in stage 1 territory below 1.6 mg/dL but SDMA stays above 18 µg/dL, the cat is staged and treated as stage 2. The same rule moves a stage 2 cat up at SDMA above 25 µg/dL, and a stage 3 cat up above 38 µg/dL [IRIS Staging of CKD, 2026 edition].
What Does Stage 1 Kidney Disease Mean?
Stage 1 means creatinine is still normal, below 1.6 mg/dL (140 µmol/L), and SDMA is normal or only mildly raised. Something else in the kidneys is abnormal instead. That might be protein in the urine coming from the kidney itself, urine your cat cannot concentrate with no other explanation, kidneys that feel or image abnormally, or both markers creeping upward across serial samples. A persistently raised SDMA above 14 µg/dL can be used to diagnose early kidney disease [IRIS Staging of CKD, 2026 edition].
Cats in stage 1 usually have no obvious symptoms at home. This stage is often found only through proactive senior bloodwork and urine testing, which is exactly why annual screening in older cats is so valuable.
There is often nothing to treat at stage 1, so the work is measuring. Your vet will check the urine protein-to-creatinine ratio and blood pressure, because those set the substage. In cats a ratio below 0.2 is non-proteinuric, 0.2 to 0.4 is borderline, and above 0.4 is proteinuric [IRIS Staging of CKD, 2026 edition]. Proteinuric cats are the group where treatment is most likely to change.
What Does Stage 2 Kidney Disease Mean?
Stage 2 is a mild, confirmed rise in creatinine, 1.6 to 2.8 mg/dL (140 to 250 µmol/L), usually with SDMA between 18 and 25 µg/dL [IRIS Staging of CKD, 2026 edition]. The bottom of that band still sits inside many laboratory reference ranges. That is part of why creatinine is a weak screening test on its own: a cat just under the upper reference limit may already have lost real excretory function. At home you may notice nothing, or only slightly more drinking and a heavier litter tray.
Many cats in stage 2 still feel and act completely normally day to day. This is frequently the stage where a diet change and more regular monitoring are first discussed, since intervening early tends to support kidney function for longer.
What Does Stage 3 Kidney Disease Mean?
Stage 3 is a moderate rise, 2.9 to 5.0 mg/dL (251 to 440 µmol/L), with SDMA typically 26 to 38 µg/dL [IRIS Staging of CKD, 2026 edition]. Signs are more likely to show at home now: clearer increases in thirst and urination, weight loss, and an appetite that comes and goes. IRIS splits this band by how the cat looks, calling a cat with no signs early stage 3 and a cat with many or marked signs late stage 3. That is one reason two cats with the same creatinine can need very different plans.
Management typically becomes more structured at this stage, often including a kidney-support diet and closer monitoring of blood pressure, phosphorus, and hydration, all guided by your veterinarian.
Owners always ask how long, and there is a real answer with a real caveat. Among 211 cats followed from diagnosis, median survival was 1,151 days for cats in the upper half of stage 2, 778 days in stage 3, and 103 days in stage 4 [Boyd and colleagues, Journal of Veterinary Internal Medicine, 2008]. The ranges inside every one of those stages ran from days to years. Those cats were also seen in 2000 to 2002, before SDMA and current renal diets. Read the medians as a description of a group, not a forecast for your cat.
What Does Stage 4 Kidney Disease Mean?
Stage 4 is a severe, confirmed rise above 5.0 mg/dL (440 µmol/L), with SDMA above 38 µg/dL [IRIS Staging of CKD, 2026 edition], and clinical signs are usually present and noticeable, including more significant appetite loss, lethargy, vomiting, and dehydration.
Stage 4 carries a real risk of a uremic crisis, where waste builds up faster than the kidneys can clear it. Call your veterinarian the same day if your cat stops eating for a full day, vomits repeatedly or develops diarrhea, becomes markedly lethargic, has ulcers in the mouth, or feels dry and tented when you lift the skin over the shoulders. Those are the signs of uremia becoming fully manifest [Merck Veterinary Manual, renal dysfunction in dogs and cats]. They need a clinic, not home care.
Even here, fluids, anti-nausea medication and appetite support make many cats considerably more comfortable. Quality of life becomes part of every recheck rather than one difficult conversation.
How Do Vets Decide Which Stage My Cat Is In?
Your vet takes fasting creatinine and SDMA on at least two occasions, with your cat hydrated and stable. Then come the substages. The urine protein-to-creatinine ratio ideally uses two urine samples collected at least two weeks apart, and blood pressure uses repeated readings rather than one [IRIS Staging of CKD, 2026 edition]. For cats, a systolic pressure under 140 mmHg is normotensive, 140 to 159 prehypertensive, 160 to 179 hypertensive, and 180 or above severely hypertensive. The top band carries the highest risk of damage to the eyes, brain, heart and kidneys.
| IRIS stage | Blood creatinine, cats | SDMA | Typical clinical picture |
|---|---|---|---|
| Stage 1 | Below 1.6 mg/dL (140 µmol/L) | Under 18 µg/dL | Usually no symptoms; another kidney abnormality present |
| Stage 2 | 1.6 to 2.8 mg/dL (140 to 250 µmol/L) | 18 to 25 µg/dL | Often no symptoms, or very subtle changes |
| Stage 3 | 2.9 to 5.0 mg/dL (251 to 440 µmol/L) | 26 to 38 µg/dL | Symptoms more likely, such as thirst and weight loss |
| Stage 4 | Above 5.0 mg/dL (440 µmol/L) | Above 38 µg/dL | Symptoms usually present; rising risk of uremic crisis |
Because this system depends on confirmed trends, a single unusual value does not immediately assign a stage, and rechecking is part of the process.
What Happens After Staging Is Complete?
Once a stage is set, it drives a schedule. IRIS recommends rechecking a stable cat with kidney disease at least every three months, and revising the stage and substages as the numbers change [IRIS Treatment Recommendations for CKD in Cats, 2026]. Diet, hydration support, phosphate control and blood pressure treatment are matched to where the cat is now, not chosen once.
A stage is not permanent. Cats can move up a stage, and the substages can move in either direction once high blood pressure or protein loss is treated. Your own vet weighs those numbers against how your cat actually looks at home, which is the part no staging table captures.
Further Reading
Cat Blood Test Results Explained is the pillar guide to the whole panel, useful for seeing where creatinine and SDMA fit into staging.
High Creatinine in Cats explains the main marker used to determine stage in more detail.
Signs of Kidney Failure in Cats covers the symptoms that often correspond to each stage described here.
SDMA in Cats covers a companion kidney marker that can flag reduced function earlier than creatinine alone, useful context at every IRIS stage.
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