Relative Supersaturation’s Role

How RSS values can help you prevent urinary crystals

If your cat has suffered from urinary crystals or bladder stones, you may have heard the term “RSS,” which stands for relative supersaturation. It refers to minerals in your cat’s urine, and it is one of the tools that can be used to assess your cat’s urine and predict crystal formation. In addition, this test is most often used by pet-food manufacturers when formulating food, especially prescription urinary diets, but it can help your veterinarian choose the best food for your cat.

To do a proper RSS, your cat may be assigned a specific diet, which means she cannot eat anything else for the specified period of time (at least two weeks). At the end of the trial, urine will be examined frequently over the course of a week or so to measure changes in RSS. These values will be considered:

Urinary specific gravity or concentration. If urine is dilute, it can handle greater amounts of minerals without crystals or stones forming in most cases. This is why cats prone to urinary problems must be encouraged to drink more.

Amounts of some minerals your cat consumes on a daily basis. These minerals include magnesium, ammonium, and phosphate for struvite problems and calcium oxalate for oxalate problems. Your cat may need a diet that limits the amounts of these minerals.

Urinary pH. Minerals will settle out to varying degrees depending upon the pH (acidity or alkalinity) of the urine.

RSS values are calculated using various software programs after looking at these three variables. An RSS lower than 1 means low likelihood of crystal formation. For struvite, an RSS between 1 and 2.5 is acceptable. For oxalate, RSS values of 1 to 10 are good. Any RSS calculated above those limits means a higher risk of crystal formation and may prompt a diet change.

RSS values can be calculated both to prevent any crystal or stone formation and to try to dissolve already formed stones through dietary management. For stone dissolution, the ideal RSS is less than 1.