Why Would My Dog's Blood Ionized Calcium Level Be Abnormal ?
Why Would My Cat's Blood Ionized Calcium Level Be Abnormal
?

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

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

To see how tests are often grouped, go here

You will find a more complete explanation of the reasons dogs and cats have abnormal blood calcium levels on this webpage: dxme-Calcium.htm

Ron Hines DVM PhD

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You Pet’s Blood Ionized Calcium Level

Free, Available,Unbound Calcium, iCa

Traditionally, veterinarians considered your pet’s blood calcium level as a single entity (unit or value). But it actually consists of three portions, ionized available free calcium (about half the total blood calcium), bound, unavailable, calcium (tied to your pets blood albumen and globulin 40-45%) and complexed calcium tied to blood anions (about 5-10%).

The free portion in the blood is the portion that is active and available at the pet’s cellular level. That is why measuring that portion separately is a better indication of the seriousness of the situation when total blood calcium is either high or low. Unlike total calcium level, ionized calcium is not affected by changes in your pet’s blood albumen levels.

Of course, the largest amount of calcium by far, in your pet’s body, remains unmeasured by blood tests - it is in its bones.

Veterinarians now feel that relying on your pet’s total blood calcium level to judge it current situation overestimates the number of pets that are truly hypercalcemic (high Ca) and under estimates the number that are hypocalcemic (low Ca). That is why a supplemental ionized blood calcium determination might be advisable when you pet is ill.

Reasons Why The Ionized Portion Of Your Pet’s Blood Calcium Might Be Low While Its Total Blood Calcium Level Remained Normal :

In chronic kidney failure, your pet’s total blood calcium value may still be within the normal range. Your pet’s kidneys are responsible for producing the active form of vitamin D. It is called calcitriol (1,25-dihydroxycholecalciferol or 1,25-dihydroxyvitamin D3).

Damaged kidneys may not produce adequate amounts of calcitriol. Without enough, it is primarily the ionized portion of your pet's blood calcium that drops. In these situations, the pet’s parathyroid glands over-produce a hormone (PTH) that causes bone throughout the body to loose calcium (secondary hyperparathyroidism). Those pets can benefit from calcitriol supplements or injections  (Calcijex) and oral phosphorus binders. (ref)

Reasons The Free or Ionized Portion Of Your Pet’s Blood Calcium Might Be High :

In older pets, various forms of cancer are the most common cause.

Over-active parathyroid glands (hyperparathyroidism), vitamin D overdose and exposure to certain rodent poisons all elevate free blood calcium levels. (see my general article on high blood calcium)

Free calcium levels are often high in growing young dogs of the larger and giant breeds.

Certain granulomatous infections (eg blastomycosis) are rarer causes of high free blood calcium levels.

Reasons Why Your Pet’s Total Blood Calcium Level Might Be Increased While its Ionized Calcium Level Remained Normal :

This can also be an effect of chronic kidney disease. It appears that that increase is due to an increase in the complexed calcium portion. But there is no need to treat the elevated total blood calcium (hypercalcemia) because the active, ionized portion of blood calcium is still normal. 

Reasons The Ionized Portion Of Your Pet’s Blood Calcium Might Be Normal While Your Pet’s Total Blood Calcium Level Remains High :

That is also sometimes the case in kidney failure and in Addison’s disease.

Reasons Both The Ionized Portion Of Your Pet’s Blood Calcium Level And The Bound Portion Levels Might Be High :

Acidosis (decreased body pH) increases ionized calcium.

A lipemic or hemolized blood sample or a dehydrated pet can also be the cause.

Thyroxin supplement, given to hypothyroid pets, can also increase its level of ionized calcium.

Reasons Both The Ionized Portion Of Your Pet’s Blood Calcium Level And The Bound Levels Might Be Low :

Malnutrition, a very low calcium diet or administration of phosphate binders to pets in kidney failure can all have that effect.

A vitamin D3 deficiency or a low blood albumen level have both been know to cause that as well.

Alkalosis (an increased body pH) will decreases ionized calcium blood levels.

Test Availability

The ionized or free calcium assay is considerably harder to perform than a simple total blood calcium determination. Ionized calcium is quite unstable. So samples that have been stored or delayed in transit can give inaccurate results. Freezing and refrigeration can change the pH of blood samples causing the ionized calcium reading to be falsely low or high, respectively. Protecting the sample from air (anaerobic collection) is also desirable to maintain accuracy. Some diagnostic laboratories feel they have overcome those problems (MichState).

Complementary Tests :

CBC and complete blood chemistry panel including blood albumen levels, PTH level, urinalysis , ACTH stimulation test, electrocardiogram for arrhythmias (=heartbeat abnormalities) , tests suggested earlier for low or high total blood calcium levels (hypocalcemia/hypercalcemia) 25-hydroxyvitamin D assay if rodent poison or other toxin ingestion or a Vitamin D deficiency is suspected

.................... DxMe