How do you compare two renal cat foods by their protein-to-phosphorus ratio?

The ratio is crude protein divided by phosphorus, with both expressed on the same basis (dry matter preferably). You then compare the ratio, but also absolute phosphorus and moisture. The calculation sometimes means asking the maker for the phosphorus figure, as it is not always on the label (FEDIAF, 2024). Expert deep dive ### How do you set the calculation up correctly? The ratio is protein over phosphorus. To compare two foods, values must be converted to the same basis, ideally dry matter, because wet and dry cannot be compared as fed (one holds about 80% water, the other 8 to 10%). Phosphorus is not a mandatory analytical constituent, so it often has to be obtained from the maker (FEDIAF, 2024). Without the phosphorus value the ratio cannot be set: it is the first figure to gather. ### Which traps should you avoid in the comparison? Comparing ratios calculated on different bases (as fed versus dry matter) distorts everything. Surprising fact: two foods with an identical ratio can have very different absolute phosphorus levels, one suiting a diseased kidney and the other not, depending on protein level. For a CKD cat, low absolute phosphorus stays the decisive criterion, with the ratio as a complement. The final diet is validated with the vet, who factors in the stage and blood work. Comparison table | Comparison step | Data to gather | Watch-out | |---|---|---| | Convert to dry matter | each food's moisture | do not compare as-fed with dry | | Obtain phosphorus | label or maker | often missing from the label | | Calculate the ratio | protein / phosphorus | same basis for both | | Read absolute phosphorus | level on dry matter | decisive in CKD | Petipedia's take Petipedia details the ratio calculation and its basic comparison traps, without ranking products or replacing veterinary analysis.