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Overview :
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.
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