Stewart SID / strong ion gap (SIG)
Stewart strong ion difference / strong ion gap (Stewart 1983; Figge 1992): the unmeasured-anion burden from a full electrolyte panel.
Open the calculator → Runs in your browser. Data leaves only if you deliberately send a problem report from the interactive tool.
Example
- Sodium
- 140 mEq/L
- Potassium
- 4 mEq/L
- Ionized calcium
- 2.4 mEq/L
- Ionized magnesium
- 1.0 mEq/L
- Chloride
- 100 mEq/L
- Lactate
- 2 mEq/L
- Bicarbonate
- 14 mEq/L
- Albumin
- 4.0 g/dL
- Phosphate
- 4 mg/dL
Result: SIG 17.8 mEq/L (apparent SID 45.4, effective SID 27.6): above ~2 mEq/L, suggesting unmeasured strong anions (ketoacids, sulfate, citrate, salicylate).
The tool opens with these values already filled in. Replace them with your own.
What the result means
- Reading
- a SIG above ~2 mEq/L suggests unmeasured strong anions (ketoacids, sulfate, citrate, salicylate); at or below ~2 mEq/L none are flagged.
What you enter
- Sodium
- Potassium
- Ionized calcium
- Ionized magnesium
- Chloride
- Lactate
- Bicarbonate
- Albumin
- Phosphate
How this is calculated
- Formula
- apparent SID = (Na + K + Ca + Mg) - (Cl + lactate); effective SID = HCO3 + albumin charge + phosphate charge (Figge weak-acid charges at pH 7.4); SIG = apparent - effective.
Stewart PA. Modern quantitative acid-base chemistry. Can J Physiol Pharmacol. 1983;61(12):1444-1461; albumin/phosphate charge per Figge J, Mydosh T, Fencl V. J Lab Clin Med. 1992;120(5):713-719. Read the source ↗
A reference and educational tool. Not medical, legal, or financial advice, and not a substitute for clinician judgment.