Chronic kidney disease (CKD) is staged with IRIS numbers, and the diet follows the stage, not the other way round. Stage 1 and 2 cats usually need phosphorus control and a palatable wet food; stage 3 and 4 cats need 26 to 32 percent protein on a dry-matter basis, a phosphate binder once phosphorus stays above target, and fluids at 10 to 20 mL/kg per session. Every one of those numbers is set by the vet's prescription and stage.
The four IRIS stages in numbers
Staging uses creatinine from 2 or more samples in a stable, well-hydrated cat, plus SDMA, urine specific gravity and proteinuria. Creatinine alone misclassifies cats with low muscle mass as stage 1 or 2, which is why SDMA now sits in the table. Conversions: 1.6 mg/dL is 141 µmol/L, 2.8 mg/dL is 250 µmol/L, 5.0 mg/dL is 442 µmol/L.
| IRIS stage | Creatinine (mg/dL) | SDMA (µg/dL) | Phosphorus target (mg/dL) | Protein (% DM) | Fluids |
|---|---|---|---|---|---|
| 1 | below 1.6 | below 18 | 2.7 to 4.6 | 28 to 35 | none routine |
| 2 | 1.6 to 2.8 | 18 to 35 | 2.7 to 4.6 | 26 to 32 | 0 to 2 per week |
| 3 | 2.9 to 5.0 | 36 to 54 | 2.7 to 4.6 | 26 to 30 | 2 to 3 per week |
| 4 | above 5.0 | above 54 | 2.7 to 5.0 | 26 to 28 | daily to every other day |
Protein at 26 to 32 percent: the trade-off
Renal diets sit at 26 to 32 percent protein on a dry-matter basis, while maintenance foods run 30 to 45 percent. Restriction lowers the nitrogen load the kidneys must excrete and reduces proteinuria, but it also costs muscle. A cat that loses 5 percent of body weight in 3 months is losing lean mass, and that predicts outcome at least as strongly as the creatinine value. Stage 1 cats with normal blood pressure and no proteinuria do not need restriction yet, and 30 to 35 percent with phosphorus controlled is reasonable. Stage 3 and 4 cats usually do better at 26 to 30 percent with enough calories, because a cat eating 40 percent less of a perfect diet gains nothing.
Weigh every 2 weeks and score muscle monthly. If weight drops more than 5 percent in 4 weeks, raise protein by 2 to 3 points before raising calories.
Phosphorus: the lever that matters most
The target is 2.7 to 4.6 mg/dL (0.9 to 1.5 mmol/L) in stages 2 and 3, and 2.7 to 5.0 mg/dL in stage 4. A renal diet alone typically pulls phosphorus down 0.5 to 1.0 mg/dL within 3 to 6 weeks. Recheck 4 weeks after any diet change; a binder goes in when phosphorus is still above target on the diet, not before, and never as a substitute for the diet.
Binders: when and how much
Aluminium hydroxide at 30 to 90 mg/kg per day divided across meals, or calcium carbonate at 60 to 90 mg/kg per day, mixed into food. For a 4 kg cat that is 120 to 360 mg a day in 2 or 3 doses; every meal should carry binder, because it only binds phosphorus in the food it meets. Never give it on an empty stomach. Watch for constipation at the top of the aluminium range, and for hypercalcemia with calcium-based products.
Subcutaneous fluids: volume, rate, frequency
A typical start is 10 to 20 mL/kg per session, 2 to 3 times a week, which is 40 to 80 mL for a 4 kg cat, rising to 100 to 150 mL in stage 4 cats that are clinically dehydrated. Use a 20 to 22 gauge needle for 60 to 100 mL and an 18 gauge above 150 mL. Warm the bag to about 38 °C, give over 5 to 15 minutes into the scruff or the side of the chest, and rotate sites. Daily fluids suit some stage 4 cats and harm cats with heart disease, so that call needs an echo and a vet rather than a schedule from a forum.
Treats, water and food temperature
Treats should stay under 10 percent of daily calories, about 20 kcal for a 4 kg cat eating 200 kcal a day. A single 3 g soft treat runs 10 to 12 kcal, so 2 a day is the ceiling. Stop the phosphorus-heavy extras first: 30 g of cheese carries about 150 mg of phosphorus, close to a whole day's allowance at stage 3, and canned fish adds sodium that pushes blood pressure. Warm renal food to 37 °C, where aroma release peaks, and offer 4 meals of 30 to 40 g instead of 2 large ones.
Water matters as much as food in stages 3 and 4. A cat on dry renal kibble alone has to drink 200 to 250 mL a day to stay ahead, and most do not. Stirring 20 to 30 mL of warm water into each meal, or moving to the wet version of the same diet, is easier than persuading a cat to drink.
What to check every 3 months
Creatinine, SDMA, phosphorus, potassium, urine specific gravity, UPC and blood pressure. Potassium below 3.5 mmol/L is common in stages 3 and 4 and usually responds to 2 to 4 mEq/day of potassium gluconate. Systolic pressure above 160 mmHg on 2 visits needs treatment. A haematocrit below 20 percent means anemia of CKD and may need darbepoetin.
When to call the vet today
No food for 24 hours, vomiting more than 2 times, drinking above 100 mL/kg per day, sudden blindness, or a cat that cannot stand. A stage 3 cat that stops eating for 2 days can be in a crisis within 72 hours. Kidney function declines with age, so a cat over 10 should have creatinine and SDMA checked yearly; the age chart at cat age converts years to life stage.
First week on a renal diet
- Ask for the stage in writing, with the creatinine and SDMA values.
- Switch over 14 to 21 days at 10, 25, 50, 75 and 100 percent.
- Feed wet if the cat will take it, in 3 to 4 small meals.
- Recheck phosphorus and potassium 4 weeks after the switch.
- Weigh weekly; a 2 percent drop triggers a protein and calorie review.
Home-cooked renal food is workable but easy to get wrong on phosphorus and taurine; the arithmetic is at homemade cooked diet recipes, and the filtration basics behind the staging numbers are at kidney function.
Frequently asked
Can a stage 2 cat stay on normal food?
Often yes, if phosphorus, blood pressure and proteinuria are controlled and the cat is eating well. Many vets still switch to a renal diet early because it is easier to prevent a phosphorus rise than to reverse one, and the transition takes 14 to 21 days either way.
How do I know the fluids are helping?
Look at 3 markers over 2 to 4 weeks: appetite, urine output and weight. A cat that gains 100 to 200 g of rehydration, eats more and stops hiding is responding; one that gains weight but develops a cough or laboured breathing may be overfilled and needs a recheck.
Is a homemade renal diet cheaper and better?
It can cost 30 to 50 percent less than prescription cans, but it needs a board-certified nutritionist's recipe to hit phosphorus, calcium, potassium and taurine targets together. Improvised recipes commonly land at 0.8 to 1.2 percent phosphorus on a dry-matter basis, which is far above the target.