What protein-to-phosphorus ratio should you target for a cat with CKD?

The protein-to-phosphorus ratio (protein divided by phosphorus) is an indicator: the higher it is, the more protein the food delivers per unit of phosphorus. In healthy cats a ratio above 35 with phosphorus under 1.1% is often cited. In CKD the priority stays low absolute phosphorus (renal diets often cut it to about 0.3 to 0.5% on dry matter), via a prescribed diet (feline nutrition literature; IRIS, 2023). Expert deep dive ### What does the ratio really measure? The protein-to-phosphorus ratio compares the protein level with the phosphorus level. A low ratio signals excess phosphorus relative to protein, often from bone-rich ingredients or added phosphates. A high ratio means the opposite: plenty of useful protein for little phosphorus. The metric was popularised in France as a way to judge a food's mineral quality. For a healthy adult cat, a ratio above 35 and phosphorus no higher than 1.1% on dry matter are commonly cited markers, though these are popular heuristics, not homologated clinical targets. ### How do you use it in a renal context? In CKD the ratio is only a screening indicator, not an approved clinical target. What matters first is the absolute phosphorus level, which renal diets often cut to about 0.3 to 0.5% on dry matter, plus protein quality. Surprising fact: a food can show a flattering ratio while still being too high in absolute phosphorus for a diseased kidney, which is why the raw phosphorus figure outranks the ratio. The stage-appropriate diet is chosen with the vet, not on the ratio alone. Comparison table | Indicator | Healthy-cat marker | Priority in CKD | |---|---|---| | Protein-to-phosphorus ratio | above 35 | secondary indicator | | Phosphorus on dry matter | under 1.1% | often 0.3 to 0.5% (renal diet) | | Protein quality | high | high, adequate level | | Final decision | case by case | prescribed renal diet | Petipedia's take Petipedia treats the ratio as a useful but secondary reading tool in CKD, where absolute phosphorus and veterinary advice come first.