Stages of Kidney Disease in Cats: What Each IRIS Stage Means

A veterinarian has put a number between 1 and 4 on your cat’s kidneys, and that number is an IRIS stage. It answers one question: how much of the kidneys’ waste-clearing capacity has already been lost, scored from confirmed fasting bloodwork. The scale comes from the International Renal Interest Society, a group of veterinary nephrologists who built it so vets anywhere could compare notes on the same yardstick. It is a planning tool, and a good one: it tells your veterinarian how closely to watch and what to treat first.

What it is not is a prediction. 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. Chronic kidney disease is common in cats and usually progresses slowly, which is worth holding onto. A stage tells a veterinarian what to do next, and remarkably little about how long.

The Whole Scale, on One Screen

Stage 1 is the odd one, because creatinine is still normal and SDMA, a second kidney marker read from the same blood sample, is normal or only mildly raised. Something else in the kidneys is abnormal instead. That might be protein leaking into the urine from the kidney itself, or urine that stays watery when it should be concentrated with nothing else to explain it. It might be kidneys that feel abnormal to your vet’s hands or look abnormal on a scan, or both markers creeping upward across repeat samples. A persistently raised SDMA above 14 µg/dL can be used to diagnose early kidney disease. Cats here usually have no symptoms at home, so stage 1 turns up only through proactive senior bloodwork and urine testing, the best argument there is for yearly screening in older cats. There is often nothing to treat yet; the work is measuring.

Stage 2 is a mild, confirmed rise in creatinine, and the bottom of that band still sits inside many laboratory reference ranges. That is why creatinine is a weak screening test on its own: a cat just under the upper reference limit may already have lost real kidney function. At home there may be nothing to see, or only a little more drinking and a heavier litter tray. Many act completely normally day to day. A clinical renal diet usually first comes up here: worth considering at stage 2, recommended from stage 3 on. More regular monitoring starts here too.

By stage 3 the signs are more likely to show at home: clearer increases in thirst and urination, weight loss, and an appetite that comes and goes. IRIS splits this band by how the cat actually looks, calling a cat with no signs early stage 3 and one with many or marked signs late stage 3. The kidney diet is now recommended rather than optional, and blood pressure, phosphorus and hydration get watched more closely.

At stage 4 the signs are usually present and noticeable, including more significant appetite loss, lethargy, vomiting, and dehydration. This is the band owners dread reading about, so it is worth saying plainly what can still be done. Fluids, anti-nausea medication and appetite support make many cats considerably more comfortable, and quality of life becomes part of every recheck rather than an afterthought.

The Signs That Need a Clinic Today

The risk that sets stage 4 apart is a uremic crisis (uremia comes from the Greek words for urine and blood), where waste builds up faster than the kidneys can clear it. That is the point at which kidney numbers become dangerous, and a cat can go downhill within a day. These are the signs that uremia has taken hold, and they need a clinic rather than home care. Call your veterinarian the same day if you see any of them.

A Stage Has to Be Earned Before It Means Anything

A stage only means something if it was arrived at properly. A cat is placed in a stage using fasting creatinine or SDMA, or preferably both, measured on at least two occasions in a hydrated, stable patient. Two things follow from that, and both are easy to miss. Staging happens after kidney disease has already been diagnosed, so a raised creatinine or SDMA on its own is not a diagnosis of kidney disease. And a dehydrated or acutely unwell cat cannot be staged at all until it is stable, because dehydration alone pushes creatinine up. That cat gets treated first and staged afterwards, not labeled on one bad day at the clinic.

Creatinine is cheap, available everywhere and easy to trend, but it 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 available. When the two disagree there is a rule. 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.

The Two Substages That Change the Plan

The stage number on its own does not finish the job, which is why two cats at the same creatinine can end up on quite different plans. IRIS adds two substages on top of the stage, one for protein leaking into the urine and one for blood pressure. Those are often the parts that change what your cat is prescribed.

Take the protein substage first. A urine protein-to-creatinine ratio compares protein leaking out against how concentrated the urine is, and under the substage cutoffs for cats a result below 0.2 is non-proteinuric, 0.2 to 0.4 is borderline, and above 0.4 is proteinuric, meaning real protein loss. Proteinuric cats are the group where treatment is most likely to change. That ratio ideally uses two urine samples collected at least two weeks apart.

Blood pressure is the other one, and it is read repeatedly rather than once. For cats, a systolic pressure under 140 mmHg is normal, 140 to 159 counts as prehypertensive, 160 to 179 as hypertensive, and 180 or above as severely hypertensive, rising degrees of high blood pressure. The top band carries the highest risk of damage to the eyes, brain, heart and kidneys. A stage 2 cat who is proteinuric and severely hypertensive therefore needs more attention than a stage 3 cat who is neither, and the substages are how that difference gets written down.

The Trend Says More Than the Stage

The question every owner wants answered is how long, and there is a real answer, with a caveat. Boyd and colleagues’ 2008 study in the Journal of Veterinary Internal Medicine followed 211 cats 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. The ranges inside each of those stages ran from days to years. Those cats were also seen between 2000 and 2002, before SDMA testing existed. Read the medians as a description of a group, not a forecast for one cat.

What a stage reliably buys is a schedule. A stable cat with kidney disease should be rechecked at least every three months, with the stage and substages revised as the numbers change, so diet, hydration support, phosphate control and blood pressure treatment match where the cat is now rather than being chosen once and left alone.

And a stage is not permanent, which cuts both ways. Cats can move up, kidney values can sometimes come back down when something treatable such as dehydration was pushing them up, and the substages can move in either direction once high blood pressure or protein loss is treated. I would rather act on a trend than a label. Your own vet weighs those numbers against how your cat actually looks at home, which is the part no staging table captures.