Phosphorus (PHOS)

What it measures

Phosphorus is a mineral needed for bones and energy metabolism, and it is removed mainly by the kidneys, so it tends to build up when kidney function declines. In cats with chronic kidney disease, controlling blood phosphorus is a key part of care.

What a high value may mean

The most common cause is reduced kidney filtration. Other possibilities: young, growing animals naturally have higher levels; hyperthyroidism in cats; eating rat poison that contains vitamin D; and phosphate enemas (cats need special caution). Hemolysis during the blood draw can also cause a false high.

What a low value may mean

Possible causes include losses from vomiting or diarrhea, feline hepatic lipidosis (fatty liver), cats that have just started tube feeding, and insulin or glucose treatment, which moves phosphorus into cells. A severely low level needs veterinary treatment.

Usually read together with

Calcium, urea nitrogen, and creatinine.

When to follow up

For cats with confirmed chronic kidney disease, IRIS recommends stage-based limits for blood phosphorus, usually reached with a prescription kidney diet and, if needed, a phosphate binder. Recheck regularly against the target your vet sets. To see which food ingredients relate to phosphorus, see the phosphate additive and high-phosphorus ingredient entries in this site's ingredient encyclopedia.

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