Serum and plasma phosphorus levels in blood mainly depends on intestinal uptake and urine excretion.
Kidneys have a key role in maintaining serum phosphorus levels, as they can either increase or reduce the quantity excreted with urines. In case of a dog or a cat fed with a high-phosphorus diet, kidneys promote its excretion and the opposite happens with a low-phosphate diet.
In cats and dogs affected by chronic kidney disease, also in the early stages, it’s difficult for the kidney to maintain the phosphate balance because as the kidney function declines, patients tend to phosphorus accumulation: this is called hyperphosphatemia.
In both dogs and cats with renal disease, hyperphosphatemia is mainly caused from a diminished phosphorus excretion (phosphates) and, to a lesser extent, it is consequence of a high acidity of blood (metabolic acidosis).
Hyperphosphatemia is often accompanied to hypocalcemia (low calcium levels in the blood serum), which leads to the increase of parathyroid hormon (PTH) as an attempt to correct calcium concentration in serum.
If hyperphosphatemia is not treated, PTH can be excessively secreted leading to renal secondary hyperparathyroidism, responsible for bone demineralization, renal interstitial and soft tissue mineralization.
Studies in dogs affected by chronic kidney disease have shown the efficacy of a low-phosphate diet (0.4% phosphorus on dry matter) in slowing the progression of renal damage and reducing calcium deposition in the kidney.
Patients in IRIS stage 2, 3 e 4 can show hyperphosphatemia that can be diagnosed testing phosphorus levels in blood and correlating results to the patient’s IRIS stage. In stage 2, dogs and cats are hyperphosphatemic with serum phosphorus above 4.6 mg/dL, in stage 3 with a phosporus level above 5 mg/dL and, finally, patients in stage 4 in case of phosphorus above 6,0 mg/dL.
First therapeutic approach in case of hyperphosphatemia of renal origin is administering a renal diet, with a low-phosphate level. It is recommended to check serum phosphorus after 2-4 weeks: diet is efficacious if serum phosphorus is below 4.6 mg/dL in IRIS stage 2, 5.0 mg/dL in stage 3 and below 6.0 mg/dL in stage 4. In case the diet alone is not working, it will be necessary to introduce phosphate binders.
| IRIS stage | Phosphorus(mg/dL) | Therapy |
| 1 | No | |
| >4.6 | Diet ± Binders | |
| 3 | >5.0 | Diet ± Binders |
| 4 | >6.0 | Diet ± Binders
|
Phosphorus-binding agents should be given together with meals or within 2 hours of feeding to maximize their binding of dietary phosphorus. Commonly employed oral phosphorus binders include aluminum hydroxide, calcium carbonate, and calcium acetate. The starting dosage of these phosphorus binders is approximately 60-90 mg/kg/day, usually divided twice, and the dosage should be adjusted by periodic evaluation of the serum phosphorus concentration. Calcium salts may be superior to other intestinal phosphate binders if ionized calcium is moderately to severely decreased, which is common in the later stages of the chronic kidney disease. Animals should be monitored for development of hypercalcemia whenever phosphorus binders containing calcium are used, especially if calcitriol is being administered concurrently.
Pharmacross NormaPhos® PLUS contains calcium carbonate and chitosamine, with Vit.C and folic acid. As a phosphate binding agent, calcium carbonate is considered safe for long term use while chitosamine is a naturally occurring substance that enhances phosphate binding of calcium carbonate and binds uremic toxins, reducing their absorption in the bloodstream. Finally, Vit.C shows antioxidant properties and the adequate content of folic acid supports the red blood cell function.
