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
The Anion Gap measures the difference between the amount of the major positively charged cations (sodium and potassium) in your dog or cat ’s blood minus the amount of negatively charged anions (chloride and bicarbonate).
Your veterinarian calculates the anion gap by subtracting the amount of the most common anions from the amount of the most common cations in your pet’s blood. That number should be a positive number (between about 8 and 33, some say 15-25).
The number is given in millieqivalents per liter of blood (mEq/L).
This test is an indicator of your pet’s acid:base balance, its blood pH and its bicarbonate buffer reserves. It is critical that the acidity (pH) of your dog or cat ’s body abe kept within a tight range by the continual presence of carbonate (=bicarbonate=HCO3-) in its blood and the pet’s kidney’s ability to discharge excess chloride in urine while conserving bicarbonate.
When your pet's AG number is abnormal, it is usually because it is a higher number than it should be. A high AG value means your pet suffers from metabolic acidosis – a condition in which the levels of acid in its body are increased (ie blood pH is too low). That is usually because your pet is deficient in bicarbonate – either because it was consumed (used up) or was lost from the body. So the treatment for a high anion gap is generally intravenous sodium bicarbonate (aka baking soda).
The most common cause in mature and older pets is chronic kidney disease. I mentioned that your dog or cat ‘s kidneys were chiefly responsible for keeping the animal’s blood pH in a narrow range. So not surprisingly, damaged kidneys or any sort of obstruction to urine flow will raise the pet’s anion gap. Sudden kidney failure (as in antifreeze poisoning and other causes of ARF), such as occasionally occurs with leptospirosis and other infections can have the same effect.
Probably the next most common causes are urinary tract blockage, as often occur in FUS in cats, oxalate (ref1, ref2) or struvite stones blocking the urinary system or due to spinal cord damage (neurogenic bladder).
The next most common cause is probably poorly managed diabetes. When your pet’s blood sugar levels are not under control, the end result is metabolic acidosis – in this case, lactic acidosis and ketoacidosis as those products build up in the pet’s blood stream. A similar effect occurs in starvation (a cause of ketoacidosis as well).
All of the health problems that negatively affect blood flow and oxygenation throughout your pet’s body can elevate the pet’s anion gap.
One of them is the circulatory collapse that accompanies septic shock. The sluggish blood flow that occurs in pets in heart failure (hypoxia) can do the same.
All problems that prevent proper oxygen intake (respiratory distress, pneumonia, prolonged surgical anesthesia) can increase anion gap.
Health problems that cause muscle rigidity or damage muscle fibers can lead to a high anion gap. Those can be related heat stroke (hyperthermia) seizures or extreme exertion when muscle becomes deficient in oxygen (muscle hypoxia, lactic acidosis, aka exertional rhabdomyolysis).
Another cause of an increased anion gap is poisoning with ethylene glycol antifreeze. That occurs both because of the effects of that chemical on the pet’s body and the severe damage it does to the pet's kidneys.
Salicylate medications (e.g. aspirin, oil of wintergreen, etc.) can cause an increase in anion gap through their effects on the kidneys as can overuse of urinary acidifiers.
A deficiency in aldosterone, as occurs in Addison’s disease can also cause a mild increased in anion gap.
Dogs with babesia infections occasionally have an increased anion gap. I do not know why.
The most common cause is a low blood albumin level. Quite rarely, a tumor of the pet’s immune cells (multiple myeloma) in which large amounts of globulin protein are released into the pet's blood stream can have the same effect.