Why Is The Osmolality (or Osmolarity) Of My Dog Or Cat's Blood Abnormal ?













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

The Osmolality Or Concentration Of Your Pet’s Blood (The difference between osmolarity and osmolality is insignificant)

Your pet’s blood osmolality value tells your veterinarian if the concentration of dissolved particles within it is elevated, normal or low.

The blood of all living things must stay very uniform in its concentration of dissolved ingredients. Your dog or cat's blood osmolality is determined by a simple mathematical formula that uses the concentration of dissolved sodium (Na+) glucose and blood urea nitrogen (BUN) found in your pet’s blood sample.

Basically, it tells your veterinarian if your pet’s blood is too concentrated or too dilute. In healthy pets, their kidneys keep the concentration of blood's dissolved ingredients under tight control by conserving water when it is needed and sending excess water out in the urine when there is too much.



This occurs in relation to the amount of ADH (anti-diuretic hormone) the pet’s hypothalamic region of its brain releases as it constantly monitors the body’s hydration. If your pet’s osmolality wanders outside its normal range, cells throughout its body will not function properly or maintain their normal shape. (There is a minor difference between blood osmolality and blood osmolarity, but it is not worth pondering over.)

Knowing your pet’s blood osmolality (either by calculating it or observing its blood Na, K, Cl and BUN levels) is critical to deciding the proper ingredients for IV fluids your vet will give in emergency, life-threatening situations.

Reasons Why Your Pet’s Osmolarity Values Might Be Elevated (Hyperosmolality, Hypertonic Blood) :

The most common cause is dehydration. That can be due to fluid loss due to diarrhea, vomiting, fever or a refusal to drink.

A much rarer cause is a form of diabetes (D. insipidus) where a hormone (ADH) that instructs the kidneys on water conservation is lacking.

Occasionally, blood osmolality will go up in ordinary diabetes due to very high blood sugar levels or due to failing kidneys that allow blood urea levels to become too high or due to too much sodium in the blood (hypernatremia),

Your pet’s blood osmolarity can also be high in antifreeze (ethylene glycol), methanol, manitol, grain alcohol and other poisonings. But that won’t be reported if its blood osmolarity was determined based on it blood Na, Glucose and BUN values as it usually is. To reveal those causes, an osmolal gap test needs to be run.

Reasons Why Your Pet’s Osmolarity Values Might Be Low (Hypo-osmolality, Hypotonic Blood) :

Too much water in your pet’s body - as would occur when excessive IV fluids were given can lower blood osmolality.

Excessive corticosteroid medications or too little sodium and potassium in the blood (hyponatremia, hypokalemia) as occasionally occurs on high doses of diuretics can both lower your pet's blood osmolality.

Any health condition that can cause water retention, such as heart or liver failure, bacterial endotoxin release or prostaglandins that impairs kidney function will also cause reading to be low.

Complementary Tests :

Urinalysis including urine specific gravity (isosthenuric values?) CBC/WBC and blood chemistry values, further tests based on those findings