Indications for performing this test:  This value is calculated from the results of a standard serum chemistry panel.  Anion gap is useful in determining the cause of acid-base abnormalities.

Calculation:

   Anion gap is calculated as shown below:

                 [(Na+  +  K+  + unmeasured cations) = (Cl-  +  HCO3-  + unmeasured anions)]

   If you rearrange the above equation to isolate the unmeasured ions:

                [(Na+  +  K+) - (HCO3-  +  Cl-)] = unmeasured anions - unmeasured cations

   Because the unmeasured cations in the body tend to be constant, any increase in anion gap is usually due to unmeasured anions.  These are usually in the form of organic acids and reflect a titrational metabolic acidosis.  Thus, a titrational metabolic acidosis is caused by elevation of organic acids in the body.  Causes include: 

• Lactic acid in shock

• Grain overload

• Ketone bodies in ketosis and starvation   

• Organic poisons such as ethylene glycol and metaldehyde

 

A normal anion gap in dogs and cats is 15-25.

A normal anion gap in other species is 10-20.

For more information . . .

• Veterinary Laboratory Medicine, Duncan, Prasse, and Mahaffey page 99-100.
• Veterinary Laboratory Medicine, Interpretation & Diagnosis.  Meyer & Harvey, pages 246.