Strong Ion Difference Calculator
Calculate the apparent strong ion difference (SIDa) from sodium, potassium, and chloride using the Stewart simplified approach. Instant interpretation against the 38–42 mEq/L reference range — a low SID flags metabolic acidosis, a high SID flags metabolic alkalosis.
How the strong ion difference works
The strong ion difference (SID) is the foundation of Peter Stewart's quantitative approach to acid–base chemistry. Stewart classified serum ions as "strong" — completely dissociated at physiologic pH (Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, lactate⁻) — versus "weak" (albumin, phosphate, bicarbonate). Because the strong ions do not participate in proton transfer, the difference between total strong cations and total strong anions becomes one of only three independent variables that determine blood pH (the others being PCO₂ and total weak acid). The simplified apparent SID used at the bedside is SID = (Na⁺ + K⁺) − Cl⁻, which captures the bulk of the strong-ion balance and is what this SID calculator computes.
The SID differs from the traditional anion gap in what it omits: the AG subtracts bicarbonate and is built to detect accumulated unmeasured anions (lactate, ketones), whereas the SID intentionally excludes bicarbonate because, in the Stewart method, bicarbonate is a dependent variable — it moves in response to changes in SID, PCO₂, and weak acids. This is the conceptual heart of the strong ion difference (Stewart approach): rather than reading bicarbonate as a cause, it reads it as a consequence. A falling SID drives chloride up relative to sodium and pulls pH down (metabolic acidosis); a rising SID does the reverse. The arithmetic behind both views is reconciled in the derivation on the anion gap formula page.
What the SID result means
The normal apparent SID is approximately 40 mEq/L, with a accepted reference band of 38–42 mEq/L. The table below maps the calculator's output bands to their acid–base interpretation in the Stewart framework.
| SID (mEq/L) | Interpretation | Clinical direction |
|---|---|---|
| < 38 | Low SID | Hyperchloremic (normal-AG) metabolic acidosis — chloride gain (normal saline), diarrhea, renal tubular acidosis. |
| 38–42 | Normal | Balanced strong-ion chemistry; no independent SID-driven acid–base disturbance. |
| > 42 | High SID | Metabolic alkalosis — chloride loss (vomiting, diuretics) or sodium gain relative to chloride. |
Because the simplified SID ignores Ca²⁺, Mg²⁺, and lactate, it is a screening tool rather than a full Stewart analysis. For a complete apparent SID, add calcium, magnesium, and lactate to the strong-ion sums. Even in its simplified form, however, the SID exposes the mechanism behind many disorders that the traditional anion gap labels only descriptively — for instance, the acidosis of large-volume normal saline resuscitation is, in SID terms, a predictable fall in SID from iatrogenic chloride loading.
This calculator runs entirely in your browser — no values are transmitted or stored. Interpretation logic is reviewed against StatPearls and the Life in the Fast Lane (LITFL) acid–base resources. Results are informational only and should always be interpreted in full clinical context by a qualified clinician.