Phosphorus in cats and dogs

What it shows

According to eClinPath, 80–85% of the body’s phosphate sits in bone and less than 1% is outside the cells, so the blood value is only an imperfect window into the whole stock. The kidneys excrete phosphate, and eClinPath names reduced filtration as the most common cause of high phosphate. In early chronic kidney disease it may still be normal; it climbs in acute kidney injury and in later stages, and the Merck Veterinary Manual describes phosphorus rising with BUN and creatinine from stage 2 onward.

Phosphate also moves between blood and cells: insulin, glucose and fast breathing (respiratory alkalosis) push it into cells and lower the blood value. And the result is sensitive to how the sample was handled — see the range section below.

Higher than reference — what it may be a sign of

• Reduced kidney filtration: chronic kidney disease, acute kidney injury.
• Young, growing animals — partly because growth hormone makes the kidneys keep phosphate.
• Sample artefact: blood left unseparated for long, or kept warm; hemolysis can then add phosphate.
• Hyperthyroidism in cats.
• Too much vitamin D (rodenticides, some plants), which raises calcium and phosphate together.
• Cell breakdown: tumor lysis, severe muscle injury.
• Underactive parathyroid glands; phosphate enemas in cats.

Lower than reference

Mild decreases are common and, per eClinPath, usually insignificant. Clearer causes are diabetes and the start of insulin, which moves phosphate into cells; primary hyperparathyroidism (high calcium with low phosphate); the start of tube feeding in cats, when phosphate can fall within 12–24 hours; vomiting, diarrhea and malabsorption; Cushing’s disease in dogs; hepatic lipidosis in cats. Very low phosphate can damage red blood cells — VCA links depleted phosphorus in diabetic patients to red cell rupture and anemia.

Look at together with

• Creatinine, urea and SDMA — a high phosphate is first read against kidney function.
• Calcium — the pair together point to parathyroid or vitamin D problems; eClinPath warns that high phosphate with calcium can lead to mineral deposits in the kidneys and gut.
• Glucose — when phosphate is low in a diabetic animal.
• Age and timing of the meal — needed to read the number at all.

Units and conversion

European labs report phosphorus in mmol/L, North American labs in mg/dL. Multiply mg/dL by 0.323 to get mmol/L: 5 mg/dL is about 1.6 mmol/L. In the app each row keeps the unit printed on the form, and the chart converts results to one unit so forms from different labs line up.

Reference range

The reference range is on your lab’s form. For puppies and kittens the adult range does not fit well, so a vet keeps age in mind. Handling matters: eClinPath notes that hemolysis and delayed separation of serum from the cells can falsely raise phosphate (IDEXX lists hemolysis too); eClinPath also recommends heparin as the only suitable anticoagulant. In a healthy dog a meal can raise phosphate slightly.

Once chronic kidney disease is diagnosed, phosphate becomes something the vet tracks closely: IRIS recommendations for cats set a separate phosphate goal for each stage of the disease and advise regular rechecks of calcium and phosphate. These goals are part of managing the disease, not a reference range.

Cats and dogs

Sources

1. eClinPath (Cornell University) — Phosphate
2. IDEXX — Complete Chemistry Panel Interpretation Guide
3. IDEXX Reference Laboratories — Specimen Collection Guidelines
4. Van Vertloo L. Renal Dysfunction in Dogs and Cats. Merck Veterinary Manual
5. Whitbread TJ. Clinical Biochemistry. Merck Veterinary Manual
6. International Renal Interest Society — IRIS recommendations for CKD in cats (PDF, 2023)
7. Cornell Feline Health Center — Chronic Kidney Disease
8. VCA Animal Hospitals — Serum Electrolytes