Calcium × phosphorus product (Ca × P) in cats and dogs

What it shows

Calcium and phosphate both circulate in the blood. When there is too much of both, they can combine into insoluble calcium phosphate salts that settle in otherwise normal tissue. This is called soft-tissue (metastatic) mineralization, and it can damage the kidneys, the stomach and gut, the heart and blood vessels. Multiplying the two concentrations gives one number that grows quickly when both rise together, so a vet notices that combination at a glance.

Because it is calculated, the product adds no new information: everything in it is already in the calcium and phosphorus rows. A high product can come from very high calcium with ordinary phosphorus, from very high phosphorus with ordinary calcium, or from both moderately high. These are different situations with different causes, so the vet always looks back at the two rows.

Higher than reference — what it may be a sign of

• Chronic kidney disease — as filtration falls, the kidneys excrete less phosphate, and phosphorus rises from stage 2 onwards. In dogs and cats chronic kidney disease is the most common cause of high phosphorus.
• Vitamin D intoxication — for example from cholecalciferol rodent bait. Vitamin D increases absorption of both calcium and phosphate from the gut, and both are seen high on blood tests within 24 to 72 hours of exposure.
• Any cause of high calcium or high phosphorus — such as acute kidney injury or cancer-related high calcium — moves the product through one of its two rows.
• Sample and age effects — hemolysis and serum left standing on the cells falsely raise phosphate, and with it the product; young animals have higher phosphate than adults.

Lower than reference

A low product simply follows low calcium, low phosphorus or both. The most common cause of low total calcium is low albumin, which lowers total calcium without touching the active, ionized part. On its own a low product carries no extra meaning. One situation where vets do watch it is surgery on a parathyroid gland: a low calcium–phosphorus product is listed among the factors linked to calcium falling too low afterwards.

Look at together with

• Calcium — preferably ionized calcium, which shows the active part; total calcium is influenced by albumin.
• Phosphorus — in kidney disease it is phosphorus itself that vets follow over time.
• Creatinine, urea and SDMA — to judge the kidney function behind a high phosphorus.
• PTH — when the calcium–phosphate balance is disturbed and the cause is unclear.

Units and conversion

The product carries the units of both factors multiplied: mg²/dL² when calcium and phosphorus are in mg/dL, mmol²/L² when both are in mmol/L. Some forms and references print it simply as “mg/dL” — it is the same number. To go from mmol²/L² to mg²/dL², multiply by 12.41: calcium 2.5 mmol/L × phosphorus 1.6 mmol/L = 4.0 mmol²/L², about 50 mg²/dL² — the same as 10 mg/dL × 5 mg/dL. In the app each row keeps the unit printed on the form.

Reference range

The reference range is on your lab’s form, if the lab prints one for this row; if not, its meaning comes from the calcium and phosphorus rows behind it. For orientation, eClinPath notes that soft-tissue mineralization can result when the product is above roughly 60–80 (in mg²/dL², printed there as mg/dL), and stresses that this depends heavily on the disease causing the high calcium and phosphate. That is a rough risk zone from a textbook, not a target and not a reference range; what it means for a particular animal is for the vet to judge.

Cats and dogs

Sources

1. eClinPath (Cornell University) — Calcium
2. eClinPath (Cornell University) — Phosphate
3. Merck Veterinary Manual — Hyperphosphatemia in Animals
4. Merck Veterinary Manual — Renal Dysfunction in Dogs and Cats
5. Merck Veterinary Manual — Hypercalcemia in Dogs and Cats
6. VCA Animal Hospitals — Cholecalciferol (Vitamin D3) Rodenticide Poisoning