IRIS Kidney Staging in Cats: The Four Stages

Chronic kidney disease in cats is staged 1 to 4 by blood creatinine, and the stage sets how often the cat returns and what gets measured at each visit. The table is short. The mistakes around it are not. Two cats both at 3.1 mg/dL sit in stage 3, but one is a thin 3 kg cat with urine specific gravity of 1.012 and the other a muscular 6 kg cat at 1.040 — same number, different disease, different plan. Stage by the number first, then correct for muscle, urine concentration, protein loss and blood pressure.

The four stages, and the trap inside stage 1

Stage 1 is creatinine below 1.6 mg/dL, and it is the only stage that cannot be made on a blood test alone. It requires other evidence of kidney disease: urine specific gravity under 1.035 in a cat that is adequately hydrated, a urine protein-to-creatinine ratio above 0.4, abnormal kidneys on ultrasound, or a persistently raised SDMA. Without one of those, a creatinine of 1.5 is just a creatinine of 1.5.

Stage 2, 1.6–2.8 mg/dL: where most decisions are made

Creatinine does not leave the reference range until roughly 60–75% of kidney function is gone, so a cat in stage 2 has already lost most of its reserve. This is the stage where intervention still changes the slope. The first visit should collect urine specific gravity, UPC, blood pressure, phosphate, potassium, SDMA and a symmetric baseline weight. Recheck every 3–6 months, or every 2–3 months if UPC or blood pressure is abnormal.

Stage 3, 2.9–5.0 mg/dL: symptoms become the agenda

At this point phosphate control, potassium, hydration, nausea and appetite drive the plan more than the creatinine itself. Many nephrologists aim to hold phosphate between 2.7 and 4.6 mg/dL; potassium is supplemented when it falls below 3.5 mEq/L, typically 1–4 mEq per day in divided doses, adjusted on a recheck in 1–2 weeks. Weight is checked every visit because 5% loss in a month is a real change at this stage. Recheck interval is 2–3 months, and each visit repeats UPC and blood pressure, since proteinuria and hypertension both accelerate loss of function.

Stage 4, above 5.0 mg/dL: quality of life over numbers

Above 5.0 mg/dL the conversation shifts from slowing progression to keeping the cat comfortable. Subcutaneous fluids at 100–150 mL every 24–48 hours are common, with the volume and frequency set by the cat's heart and blood pressure rather than by habit. Antiemetics such as maropitant at about 1 mg/kg once daily, phosphate binders mixed into every meal, and appetite support carry more weight than another dose tweak. Recheck every 2–4 weeks, or weekly while unstable. It is fair to discuss the criteria for euthanasia at this stage, in daylight, rather than during a crisis at 2 a.m.

Substage with UPC and blood pressure

IRIS substaging is what turns a stage into a plan. Proteinuria: UPC under 0.2 is non-proteinuric, 0.2–0.4 is borderline and needs a repeat in 2–4 weeks, above 0.4 is proteinuric and usually treated with benazepril at 0.25–0.5 mg/kg once daily, aiming for at least a 50% fall in UPC by 4–8 weeks. Blood pressure: under 150 mmHg systolic is normal, 150–159 is borderline, 160–179 is hypertensive, 180 and above is severe and needs a retinal exam the same week. Amlodipine at 0.625 mg per cat once daily is the usual start, with a recheck in 7–14 days and a target of 120–160 mmHg.

Why one creatinine is never the stage

Dehydration can lift creatinine by 0.3 to 1.0 mg/dL and drop it back within a day of fluids. A 0.3 change inside the reference range is noise; a 0.3 change at 4.8 is a different clinical picture. Use three values across 4–8 weeks, always beside a urine specific gravity, and remember that a cat with creatinine 3.0 and urine at 1.045 is concentrating and may be stable, while one at 2.6 with 1.010 is losing the ability to concentrate.

Stage	Creatinine	Also required	Main actions	Recheck
1	under 1.6 mg/dL	USG under 1.035, UPC above 0.4, SDMA high, or abnormal imaging	Blood pressure, UPC, dental care, avoid nephrotoxins	6–12 months
2	1.6–2.8 mg/dL	Persistent azotemia, concentrated urine may still be present	Renal diet trial, phosphate, BP, UPC, weight trend	3–6 months
3	2.9–5.0 mg/dL	—	Phosphate 2.7–4.6, potassium, nausea, hydration	2–3 months
4	above 5.0 mg/dL	—	Fluids, antiemetics, binders, comfort and appetite	2–4 weeks

Diet, phosphate and the 2–4 week switch

A renal diet is the single intervention with the most consistent support at stages 2–3: protein around 26–32% of dry matter, phosphate under 0.5% of dry matter, and added potassium and omega-3 fatty acids in most formulations. The transition is where most attempts fail. Mix 25% new food for 3–4 days, then 50% for 3–4 days, then 75% for 3–4 days, warming the food to body temperature and never letting the cat go more than 24 hours without eating. If the cat refuses after 4 weeks, a phosphate binder with the existing diet is a reasonable second path — aluminum hydroxide at 30–60 mg/kg per day divided across meals, or calcium carbonate at about 90 mg/kg per day — rather than a hunger strike. Protein restriction below 26% of dry matter in stage 1 or 2 without proteinuria tends to cost muscle, not save kidney.

What to track between visits

Weigh the cat weekly on a baby scale to the nearest 10 g; note water intake from a measured bowl for one 24-hour period each month; score appetite and vomiting on a simple 1–5 scale daily; and count litter clumps, since urine volume rising from 2 to 5 per day is a real signal. Bring those numbers to the visit. For a cat under a year old being worked up for a congenital kidney problem, growth matters more than any single value, and the kitten weight tool is the easiest way to see whether the curve is flattening.

Two unrelated infections come up often enough to mention. Stress from repeated visits can trigger a flare in a cat carrying feline herpesvirus, so read the FHV long-term guide before assuming every eye discharge is new kidney-related disease. Toxoplasma can inflame the kidney in an immunosuppressed cat, but a positive IgG in a stable cat with CKD is rarely the explanation; the toxoplasmosis guide covers when titers are worth sending.

Five things to do at the next visit

1. Ask for the stage and the substage in writing: for example stage 3, proteinuric, borderline hypertensive.
2. Request urine specific gravity and UPC with every renal panel; creatinine alone cannot stage or substage.
3. Measure blood pressure before discussing diet, because hypertension changes the urgency.
4. Set the next recheck date in the room, matched to the table above, instead of "in a few months".
5. Keep a single sheet with date, weight, creatinine, phosphate, potassium and UPC, so the slope is visible.

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