Anion Gap

Approach to ABG analysis, the principles

1. Check arterial pH for net deviation
2. Assess the pattern
3. Look for associated clues
4. Assess the compensatory response
5. Check for additional indices for metabolic acidoses

If the patient has metabolic acidosis, determine whether it is normal anion gap or high anion gap metabolic acisosis. Correct the anion gap to patient's albumin level.
Look for mixed disorders e.g. mixed normal with HAG acidoses.
Important gap and other indices in ICU

1. Anion gap
2. Osmolar gap
3. Delta gap
4. Delta ratio
5. Lactate gap
6. Oxygen saturation gap
7. Urinary anion gap

Pearls and nonsense

• The absence of an anion gap does not exclude typical causes of anion gap acidosis, i.e. DKA may present without an anion gap.
• There is no identifiable cause of an HAG in 1/3 of patients.
• Large AG are most commonly DKA or lactic acidosis, less commonly ethylene glycol. Higher AGs correlate with increased severity of illness.
• Delta gap of > 2 - the presence of a mixed disorder is likely.

A reduced anion gap may be seen with :
-->Principle: decreased unmeasured anions, increased unmeasured cations and analytical errors.

• hypermagnesaemia
• hypercalcaemia
• Lithium toxicity
• Excess Immunoglobulins (multiple myeloma, intragam infusion)
• hypoalbuminaemia

Causes of elevated AG without acidosis:

• dehydration
• alkalosis
• sodium salts of unmeasured anions (citrate, lactate or acetate)
• certain antibiotics ( Na penicillin, carbenicillin)
• decreased in unmeasured cations (severe combined hypomagnesaemia, hypocalcaemia and hypokalaemia)

Define standard base excess (SBE): ?
ANION GAP: Anion gap is 'unmeasured anions, = (Na + K) - (Cl + HCO3)
Metabolic acidosis (reduced bicarb) must be associated with either an increased of the AG (HAG acidosis) or of the Chloride (NAG or hyperchloraemic) since the overall quantity of cations and anions must match bicarb loss from gut or kidney.With replacement by Cl- containing fluid will result in a non anion gap hyperchloraemic acidosis.