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Overview :
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)
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)
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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