Why Is My Dog Or Cat's Blood Phosphorus Level High or Low ?













Ron Hines DVM PhD

To see what normal blood and urine values are, go here

For an explanation of causes of most abnormal blood and urine tests, go here

To see how tests are grouped, go here

Your Pet's Blood Phosphorus Level = Phosphate, PHOS

Most of your dog or cat' s phosphorus is locked away in its bones (85%). There, it is combined with oxygen and calcium as calcium phosphate (hydroxyapatite). But a small portion circulates in your pet's blood as phosphate ion (PO4).

That portion of phosphorus/phosphate is important for proper nerve function and muscle contraction. Your pet’s kidneys, under the control of parathyroid gland hormone (PTH), help keep that circulating phosphate from rising too high. So when your pet's blood phosphate levels are abnormally elevated, failing kidneys are the most common cause.

Calcium and phosphate circulate in your pet’s blood in a teeter totter relationship; when one is high, the other is almost always low. Veterinary laboratories and in-office analytical machines almost always determined both at the same time.



Reasons Why Your Pet’s Blood Phosphorus Level Could Be Too High (Hyperphosphatemia) :

The most common cause of elevated blood phosphorus in middle aged and elderly dogs and cats is kidney failure. Every life seems to have a weakest link - one that fails due to the wear and tear of time. In us, it is often our hearts, in dogs and cats it is often their kidneys.

When blood phosphorus levels go up in younger pets, things like inherited genetic defects (eg polycystic kidney disease) blocked urinary tract (eg kidney stones), kidney infections (eg leptospirosis) or toxic ingestions of things like antifreeze, certain rodent poisons or other toxic products need to be considered.

Circulatory problems and heart disease that decrease blood flow through the dog or cat 's kidneys (decreased GFR) can also elevate blood phosphate levels.

Hyperthyroidism (usually in a cat) can also cause blood phosphate levels to increase, as can abnormalities in the level of PTH released by your pet’s parathyroid glands or the loss of those glands during neck surgery.

Young growing animals sometimes have slightly higher blood phosphate levels that what is considered normal in adults.

Phosphate is also occasionally elevated when a bone tumors (osteosarcoma) or a bone-occupying tumor (monoclonal gammaopathy) is present or some other bone-dissolving malady occurs.

Phosphorus blood levels also occasionally go up in pet’s (cats) whose pituitary gland secretes too much growth hormone (acromegaly).

Acidosis, usually as the result of uncontrolled diabetes (metabolic acidosis, diabetic ketoacidosis), can also raise your pet's blood phosphate levels.

Fleet-type phosphate enemas, particularly when the full human dose is given to a small pet, can raise blood phosphate to dangerous levels.

Blood samples that hemolized during collection or later in processing will have falsely high phosphate readings as will samples that sit too long before the serum is separated from its cellular portion.

Tissue and muscle destruction (rhabdomyolysis) from any cause can cause a temporary rise in blood phosphate levels.

Starvation, sustained vomiting and diarrhea have been associated with both high and low blood phosphate levels.

Reasons Why Your Pet’s Blood Phosphorus Levels Might Be Too Low (Hypophosphatemia) :

A low blood phosphorus level is considerably less common that an elevated phosphorus level. The causes are also less well understood.

Anything that causes your pet’s blood calcium level to go up will probably cause its blood phosphorus level to go down. That can include a recent large meal, the use of phosphate binding medications (eg aluminum hydroxide, sucralfate), corticosteroid medications or Cushing’s disease.

Certain tumors that release PTH-related peptides, feeding very low phosphorus or low vitamin D diets, insulin or glucose injection and the diuretic (furosemide) have all been associated with low blood phosphate.

Low body temperature (hypothermia) , hepatic lipidosis in cats, and some genetic kidney defects can also account for low blood phosphate levels.

High levels of bilirubin in your pet’s blood sample or the use of anticoagulants in the collection tube (needs to be a red top tube) can cause falsely-low phosphate results.

Complementary Tests :

CBC/WBC and blood chemistry panel especially for elevations in BUN and creatinine, urinalysis with urine specific gravity looking for the low sp gr of chronic kidney disease, Free T4 levels for evidence of hyperparathyroidism, blood PTH levels for evidence of hyperparathyroidism, review of your pet's diet