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.
= (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)
urate in mEq/L =
= (urate in mg/dL) * 0.059 * (10^((pH) – 5.75)) / (1 + (10^((pH) – 5.75)))
phosphate in mEq/L =
= 1 + ((PO4 in mmol/L) * (10^((pH) - 6.8)) / (1 + (10^((pH) - 6.8)))
protein in mEq/L =
= 10.3 * ((pH) – 5.66) * 0.1 * (total protein in g/dL)
• 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.
• 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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Specialty: Endocrinology, Clinical Laboratory, Emergency Medicine, Critical Care