Phosphorus Targets by CKD Stage in Cats

Phosphorus control in feline CKD comes down to 3 numbers: the blood target for the stage, the dry matter percentage in the bowl, and the milligrams of binder per meal. Get the first one wrong and the diet is guesswork; get the second wrong and a can at 80 percent moisture that reads 0.15 percent on the label is 0.75 percent dry matter.

The working targets used in most feline practice: stages 1 and 2 sit at 0.5 to 0.8 mmol/L (1.6 to 2.5 mg/dL), stage 3 goes as low as the cat will keep eating, and a renal diet holds phosphorus at or under 0.5 percent dry matter.

Three units, one number

Phosphorus is reported in mg/dL in the US and mmol/L in most of Europe and Asia, and the conversion is fixed: multiply mmol/L by 3.1 to get mg/dL, or divide mg/dL by 3.1. A result of 1.9 mmol/L is 5.9 mg/dL, which is above target for every stage, while 0.7 mmol/L is 2.2 mg/dL and inside the stage 1 to 2 band.

Food labels are the harder unit. Phosphorus on a can is given as-fed, and dry matter percentage is as-fed divided by (100 minus moisture) times 100. A pate at 78 percent moisture with 0.18 percent as-fed phosphorus is 0.82 percent dry matter; the same 0.18 percent on a dry food at 10 percent moisture is 0.2 percent dry matter. Two labels that look identical differ by 4 times.

Targets by IRIS stage

Stage is set by creatinine and by the clinical picture, not by phosphorus alone. Once the stage is known, the phosphorus target is a band, and the job is to stay inside it without the cat losing weight.

IRIS stage	Creatinine	Phosphorus target	Diet P, % DM	Recheck
1	<140 umol/L (1.6 mg/dL)	0.5-0.8 mmol/L (1.6-2.5 mg/dL)	0.45-0.55	12 weeks
2	140-250 umol/L (1.6-2.8 mg/dL)	0.5-0.8 mmol/L (1.6-2.5 mg/dL)	0.40-0.50	8 weeks
3	251-440 umol/L (2.9-5.0 mg/dL)	as low as intake allows, aim under 0.8 mmol/L	0.35-0.45	4-6 weeks
4	>440 umol/L (5.0 mg/dL)	as low as intake allows, never below appetite	0.30-0.40	2-4 weeks

Stage 3 is where the two goals collide. Pushing phosphorus from 1.4 to 0.9 mmol/L is worth a diet change; pushing it to 0.6 by adding a third binder to a cat eating 60 percent of its calories is not, because a cat that stops eating loses muscle, and muscle loss tracks survival more closely than a 0.2 mmol/L difference.

Reading a renal diet

A renal diet at 0.35 to 0.50 percent phosphorus dry matter is roughly 40 to 60 percent below a maintenance food at 0.9 to 1.2 percent. Work the daily milligram figure out from calories with a food calculator, not from the can alone. For a 4 kg cat eating 200 kcal, that is a drop from about 300 to 350 mg of phosphorus a day to 150 to 180 mg, which is often enough to move serum phosphorus 0.3 to 0.6 mmol/L within 4 to 8 weeks without any binder.

Two practical checks: a food under 0.5 percent dry matter usually also runs 26 to 30 percent protein and 0.6 to 0.9 percent potassium, and the taurine should still be at or above 0.2 percent dry matter. If a cheap renal food meets the phosphorus number but drops taurine, the trade is a cardiac risk for a kidney number.

Binders: dose, timing, and the 15 minute rule

A binder only binds what is in the gut at the same time, so it goes with food, within 15 minutes of the first bite, split across meals. Aluminum hydroxide at 30 to 90 mg/kg per day divided into 2 or 3 doses is the usual first choice; calcium carbonate at 30 to 90 mg/kg per day is second but raises calcium, so it is a poor pick if ionized calcium is already above 1.4 mmol/L.

Sevelamer at 30 to 60 mg/kg per meal and lanthanum at 20 to 40 mg/kg per day are used when aluminum causes constipation or the phosphorus will not budge. Expect a fall of 0.2 to 0.5 mmol/L over 2 to 4 weeks at a full dose. Constipation is the commonest reason to switch, and it usually appears at the top of the aluminum range.

Protein versus phosphorus

Restricting protein lowers phosphorus only slightly, because the phosphorus in food is not all bound to protein. Cutting protein from 32 to 26 percent dry matter moves total phosphorus by about 0.05 to 0.1 percent, while swapping to a renal diet moves it 0.4 percent. Early stages therefore lose more from protein restriction than they gain.

Keep protein at 26 to 30 percent dry matter in stages 1 to 3 unless proteinuria or uremic signs force less. Below 24 percent, muscle mass falls at 1 to 2 percent of body weight per month in many cats, and that is not reversible by adding phosphorus binder.

What actually moves the number

Treats are the quiet leak. Ten grams of a dry treat at 1.0 percent phosphorus dry matter adds about 90 mg, which is most of a day's allowance for a 4 kg cat on a renal diet. Cheese, liver, and most jerky are worse. If a cat needs 6 treats a day to take a pill, use a renal-friendly treat and count it.

Hydration matters for the same reason: canned food at 78 percent moisture delivers 3 to 4 times the water of dry food per calorie, and a cat on wet food usually holds phosphorus lower at the same intake. A water fountain is not a substitute for moisture in food.

Monitoring schedule

Recheck phosphorus 4 to 8 weeks after any diet or binder change, at the same laboratory, and put the date in a routine care calendar so it does not slip, because inter-lab variation on phosphorus runs 0.1 to 0.2 mmol/L. Once stable, every 2 to 3 months in stages 1 and 2, every 4 to 6 weeks in stages 3 and 4. Pair it with creatinine, potassium, and body weight, and urine protein when the cat is stable enough to spare the sample.

Do not restrict phosphorus in a kitten, a pregnant queen, or a cat recovering from a 3 week fast. Growth and repair need 0.8 to 1.2 percent dry matter, and a low-phosphorus diet in a growing animal causes poor mineralization that no later diet fixes.

• Convert every label to percent dry matter before comparing 2 foods.
• Set the stage with creatinine, then write the phosphorus target in mmol/L and mg/dL.
• Give binder within 15 minutes of the first bite, split over 2 or 3 meals.
• Recheck 4 to 8 weeks after each change, at the same lab.
• Stop tightening if intake drops below 80 percent of the calorie target.

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