Constable PD:Clinical assessment of acid-base status: Comparison of the Henderson-Hasselbach and strong ion approaches, Vet Clin
Pathol 29:115-128, 2000
Another frequently encountered concept in acid-base physiology is that of the Anion
Gap. Concentrations of anions and cations must remain equal in plasma to retain
electrical neutrality. An actual ‘anion gap’ does therefore not exist. Only certain
cations and anions are routinely measured. The ‘anion gap’ is the difference between
the unmeasured cations and the unmeasured anions.
It is used mainly in differentiating the underlying causes of metabolic acidosis. It is
calculated as follows:
Anion gap = (Na+ + K+) – (Cl- + HCO3-)
Normal anion gap = ± 12-16 = Albumin, sulphate, phosphate, lactate, ketones &
inorganic anions
Decreased anion gap (alkalosis) may be caused by:
Hypoalbuminaemia (albumin is an anionic protein)
Hyperchloremic metabolic alkalosis – unlikely in foals
¡ Increased K+: Renal failure / Addisons
¡ Decreased K+: GIT fluid losses / Renal tubular acidosis
Increased cationic proteins – unlikely in foals
¡ Polyclonal gammopathy
¡ Multiple myeloma
Overhydration
Increased anion gap (acidosis) may be caused by:
• Lactic acidosis
o Anaerobic exercise
o Grain overload
o Hypovolaemia
• Ketoacidosis
• Uremic acidosis
• Anionic Poisons
• Dehydration
Proceedings
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