The Strong Ion Gap is similar to the serum anion gap, but also incorporates measurements of anions and cations present at low concentrations. This can help identify unmeasured anions due to metabolic or toxic conditions.
Parameters:
(1) apparent strong ion difference, or [SID]a
(2) effective strong ion difference, or [SID]e
[SID]a =
= (sodium concentration in mEq/L) + (potassium concentration in mEq/L) + (magnesium concentration in mEq/L) + (calcium concentration in mEq/L) – (chloride concentration in mEq/L) – (lactate concentration in mEq/L) – (urate concentration in mEq/L)
where:
• Conversion of magnesium in mg/dL to mEq/L involves multiplying by 0.8333 (conversion of mg/dL to mmol/L is 0.4114, then multiplied times 2 to mEq/L)
• calcium corrected for serum albumin in mg/dL = (measured calcium in mg/dL) + 4 – (serum albumin in g/dL)
• ionized calcium in mmol/L = (calcium corrected for albumin in mg/dL) * 0.2495 * (0.46 – (0.2 * (pH – 7.4)))
• ionized calcium in mEq/L = (ionized calcium in mmol/L) * 2
• urate = (urate in mg/dL) * 0.059 * (10^((pH) – 5.75)) / (1 + (10^((pH) – 5.75)))
• Conversion of lactate in mg/dL to mmol/L involves multiplying by 0.111
[SID]e =
= mEq/L due to pCO2, pH, protein and phosphate =
= A + B + C
where:
• A = 1000 * 2.46 * (10^(-11)) * (pCO2 in mm Hg) / (10^((-1) * (pH)))
• B = (albumin in g/dL) * ((0.123 * (pH)) – 0.631)
• C = (PO4 in mmol/L) * ((0.309 * (pH)) – 0.469)
• phosphate in mg/dL is converted to mmol/L by multiplying by 0.323; this does not appear to be multiplied by 2
strong ion gap =
= [SID]a – [SID]e
Interpretation:
• Normally both [SID]a and [SID]e are about 40 mEq/L, with the strong ion gap = 0.
• If the gap is > 0, then there is an increase in unmeasured anions.
