- Enter the required values from a paired chemistry and arterial blood gas: Na⁺, K⁺, Cl⁻ (mEq/L), HCO₃⁻ (mEq/L), albumin (g/dL), and pH. SIDa, SIDe, and SIG update automatically.
- For full accuracy also enter lactate (mmol/L), ionized calcium (mg/dL), magnesium (mg/dL), and phosphate (mg/dL). Leaving any of these blank treats it as 0, which reduces accuracy — the result will note this.
- The tool converts each measured value to mEq/L or mmol/L internally (Ca×0.5, Mg×0.8226, albumin×10 to g/L, phosphate÷3.097), then computes SIDa − SIDe = SIG.
- Interpret the SIG: ≈ 0–2 mEq/L is normal; an elevated SIG (> ~2) signals unmeasured strong anions beyond lactate — a sensitive marker of occult organic acidosis even when the anion gap is normal.
All computation runs in your browser; no values are stored or transmitted.
When to Use
Use the Strong Ion Gap when you suspect a metabolic acidosis driven by unmeasured anions but want a quantitative, albumin-corrected measure that the conventional anion gap cannot give. The Stewart (physicochemical) approach decomposes the acid–base state into the strong ion difference, total weak acids, and pCO₂; the SIG is the residual gap once the contributions of bicarbonate, albumin, and phosphate are accounted for. A SIG near zero means the measured strong ions and weak acids fully explain the picture; an elevated SIG means strong anions you have not measured are present.
Appropriate population
Critically ill adults with metabolic acidosis — especially the septic, post-resuscitation, hypoalbuminemic ICU patient in whom the standard anion gap is unreliable. Particularly useful when the anion gap (or even the albumin-corrected anion gap) looks normal yet occult organic acidosis is suspected, and when a paired chemistry, albumin, phosphate, ionized calcium, magnesium, lactate, and arterial blood gas are all available.
When NOT to rely on it
The SIG is computation-heavy and lab-dependent: it requires an accurate albumin, phosphate, and the full electrolyte panel drawn close in time to the blood gas. Small assay errors propagate. It does not identify which unmeasured anion is present (ketoacids, sulfate, citrate, salicylate, toxic alcohols, sepsis anions) — only that a gap exists — and it does not replace clinical reasoning or a targeted toxicology and metabolic workup. Interpret in context, and use the albumin-corrected anion gap as the simpler bedside surrogate when a full panel is unavailable.
Pearls & Pitfalls
SIG sees what the anion gap misses
The SIG can be elevated even when the standard anion gap and the albumin-corrected anion gap look normal — because hypoalbuminemia lowers the conventional anion gap and can mask an acidosis. The Stewart approach corrects for albumin (and phosphate) explicitly, so SIG is the most sensitive screen for occult organic acidosis in the critically ill. Lactate is already subtracted out in SIDa, so a high SIG points to the other unmeasured anions.
The simpler bedside surrogate
When you cannot run the full Stewart panel, the albumin-corrected anion gap is the quick proxy: add roughly 2.5 mEq/L to the measured anion gap for every 1 g/dL the albumin sits below 4 g/dL. It captures most of what SIG corrects for at the bedside, though SIG remains the more complete, quantitative measure.
Pitfalls
(1) SIG is computation-heavy and lab-dependent — it needs an accurate albumin and phosphate plus the full electrolyte panel, and small assay or timing errors propagate. (2) Always interpret in context: a positive SIG flags unmeasured strong anions but does not name them. (3) Units matter — calcium and magnesium reported in mg/dL must be converted to mEq/L, albumin to g/L, and phosphate to mmol/L before the equations apply (this tool does the conversions for you). (4) Garbage in, garbage out: a sample drawn far from the blood gas, or with missing electrolytes defaulted to zero, will distort SIDa and the resulting SIG.
Why Use It
The conventional anion gap is convenient but blunt: it is distorted by albumin, can be normal in the presence of significant organic acidosis, and gives no quantitative measure of unmeasured anions. The Stewart physicochemical model reframes acid–base balance in terms of the independent variables that actually determine pH — the strong ion difference, total weak acids (Atot, dominated by albumin and phosphate), and pCO₂ — and the Strong Ion Gap distills this into a single number: how much unmeasured strong anion remains after everything you have measured is accounted for. In the hypoalbuminemic, septic, critically ill patient, where the anion gap is least trustworthy, SIG is the most sensitive screen for occult organic acidosis. It turns a vague suspicion that "the numbers don't add up" into a defensible, quantitative statement about hidden anions.
Stewart Strong Ion Gap (SIG)
Enter the electrolytes, pH, bicarbonate and albumin (all required); add lactate, ionized calcium, magnesium and phosphate for full accuracy. The tool converts units, then computes the apparent strong ion difference (SIDa), the effective strong ion difference (SIDe), and the Strong Ion Gap (SIDa − SIDe).
⚕ Stewart PA. Can J Physiol Pharmacol. 1983; Figge J et al. J Lab Clin Med. 1992. The Strong Ion Gap quantifies unmeasured strong anions after correcting for albumin and phosphate; it does not identify which anion is present and is sensitive to lab accuracy and sample timing. Interpret alongside the anion gap, lactate, and the full clinical picture. For licensed clinicians; not a substitute for individualized assessment.
Next Steps
Use the SIG value to decide whether unmeasured strong anions are driving the acid–base picture, and direct the workup.
- SIG ≤ 2 mEq/L (normal): the measured strong ions and weak acids explain the picture — no significant unmeasured anion load. Attribute the acidosis to the measured components (hyperchloremia, lactate) and treat accordingly.
- SIG 2–6 mEq/L (borderline / mildly elevated): a modest burden of unmeasured anions. Recheck a contemporaneous panel, confirm albumin and phosphate, and correlate with lactate and the clinical course.
- SIG > 6 mEq/L (clearly elevated): a substantial unmeasured strong-anion load. Pursue the cause — ketoacids, sulfate (uremia), citrate (massive transfusion / regional anticoagulation), salicylate and toxic alcohols, and the anions of sepsis — with a targeted toxicology and metabolic workup.
- Cross-check with the anion gap and albumin-corrected anion gap and the full arterial blood gas analysis; a high SIG with a "normal" anion gap is the classic occult-acidosis pattern.
Evidence & References
Formulas (Figge–Stewart)
| Quantity | Formula |
|---|---|
| SIDa (apparent, mEq/L) | Na + K + Ca + Mg − Cl − Lactate |
| SIDe (effective, mEq/L) | HCO₃ + [Alb(g/L) × (0.123 × pH − 0.631)] + [PO₄(mmol/L) × (0.309 × pH − 0.469)] |
| SIG (mEq/L) | SIDa − SIDe |
Unit conversions used
| Measured value | Conversion |
|---|---|
| Ionized calcium (mg/dL → mEq/L) | × 0.5 |
| Magnesium (mg/dL → mEq/L) | × 0.8226 |
| Albumin (g/dL → g/L) | × 10 |
| Phosphate (mg/dL → mmol/L) | ÷ 3.097 (≈ × 0.323) |
Interpretation of SIG
| SIG (mEq/L) | Interpretation |
|---|---|
| ≤ 2 (≈ 0–2, normal) | Measured ions/weak acids explain the picture — no significant unmeasured anions |
| 2–6 (borderline / mildly elevated) | Modest unmeasured strong-anion burden — recheck panel and correlate |
| > 6 (clearly elevated) | Substantial unmeasured strong anions — pursue ketoacids, sulfate, citrate, salicylate/toxic alcohols, sepsis anions |
Because lactate is subtracted out within SIDa, an elevated SIG reflects the other unmeasured strong anions, and can be high even when the conventional anion gap — distorted by hypoalbuminemia — appears normal. This makes SIG the most sensitive screen for occult organic acidosis in the critically ill.
References
- Stewart PA. Modern quantitative acid-base chemistry. Can J Physiol Pharmacol. 1983;61(12):1444–1461.
- Figge J, Mydosh T, Fencl V. Serum proteins and acid-base equilibria: a follow-up. J Lab Clin Med. 1992;120(5):713–719.
- Kellum JA. Clinical review: reunification of acid-base physiology. Crit Care. 2005;9(5):500–507. doi:10.1186/cc3789.
