Normally the serum anion gap is calculated using the main serum electrolytes. Inclusion of measurable anions present in low concentration can help identify the true gap caused by unmeasured anions.
AGc =
= (sodium concentration in mEq/L) + (potassium concentration in mEq/L) – (chloride concentration in mEq/L) – (bicarbonate in mEq/L) – (lactate concentration in mEq/L) – (urate concentration in mEq/L) – (phosphate in mEq/L) – (protein in mEq/L)
= 10.3 * ((pH) – 5.66) * 0.1 * (total protein in g/dL)
where:
• Conversion of lactate in mg/dL to mmol/L involves multiplying by 0.111
• Phosphate in mg/dL is converted to mmol/L by multiplying by 0.323; this does not appear to be multiplied by 2.
Interpretation:
• The anion gap including low concentration anions gives a negative value (around –16 mEq/L). Inclusion of calcium and magnesium (see previous section) into the equation would give values closer to neutrality.
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