Ion Balance Calculator

SID = (Na⁺ + K⁺ + Ca²⁺ + Mg²⁺) − (Cl⁻ + Lactate)

The Strong Ion Difference (SID) uses the Stewart approach to acid-base physiology. It calculates the balance between strong cations and strong anions to determine the metabolic acid-base status independent of bicarbonate.

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Interactive Tool

Calculate Strong Ion Difference

Enter your ion values for instant SID calculation with clinical interpretation.

Ion Values

All values in mEq/L (mmol/L)

Results

Enter values and click Calculate to see your results

Formula & Calculation

Strong Ion Difference Formula

The Stewart approach to acid-base balance uses strong ions as independent determinants of pH.

Apparent SID

SID = (Na⁺ + K⁺ + Ca²⁺ + Mg²⁺) − (Cl⁻ + Lactate)

The apparent SID sums all strong cations and subtracts all strong anions. Strong ions are those that are fully dissociated at physiological pH. The normal apparent SID is approximately 40 mEq/L.

Effective SID

SIDeff = HCO₃⁻ + Albumin charge + Phosphate charge

The effective SID represents the charges that must be balanced by weak acids (bicarbonate, albumin, phosphate). When apparent SID ≠ effective SID, the difference is the Strong Ion Gap (SIG), indicating unmeasured strong ions.

Step-by-Step Calculation

1
Sum the strong cations. Na⁺ + K⁺ + Ca²⁺ + Mg²⁺ = 140 + 4 + 4.5 + 1.5 = 150 mEq/L.
2
Sum the strong anions. Cl⁻ + Lactate = 104 + 1.0 = 105 mEq/L.
3
Calculate the SID. 150 − 105 = 45 mEq/L. This space must be filled by weak acids (HCO₃⁻, albumin, phosphate).
4
Interpret. Normal SID is 36–44 mEq/L. Low SID → acidosis. High SID → alkalosis.

Live Calculation Preview

Updates in real-time as you change values above.

Understanding the Basics

What is Strong Ion Difference?

The Stewart approach provides a physicochemical framework for understanding acid-base balance.

Definition

The Strong Ion Difference (SID) is the net charge difference between all strong cations and strong anions in the blood. "Strong" ions are those that are completely dissociated at physiological pH — they don't participate in buffering reactions.

According to Peter Stewart's approach, three independent variables determine blood pH: SID, total weak acid concentration (Atot, mainly albumin), and pCO₂. Bicarbonate is a dependent variable — it adjusts based on these three factors.

Clinical Importance

The Stewart approach can identify acid-base disorders that traditional analysis misses, especially in ICU patients with complex metabolic derangements. It separates chloride-related acidosis from unmeasured anion acidosis and explains dilutional acidosis from IV fluids.

Na⁺
K⁺
Ca²⁺
Mg²⁺
Cl⁻
Lactate
SID
Reference Values

Strong Ion Difference Normal Range

Reference values for the SID and its component ions.

IonSymbolNormal RangeUnitType
SodiumNa⁺136 – 145mEq/LStrong Cation
PotassiumK⁺3.5 – 5.0mEq/LStrong Cation
CalciumCa²⁺4.0 – 5.0mEq/LStrong Cation
MagnesiumMg²⁺1.3 – 2.1mEq/LStrong Cation
ChlorideCl⁻98 – 106mEq/LStrong Anion
LactateLac⁻0.5 – 2.0mEq/LStrong Anion
SID (Apparent)SID36 – 44mEq/LCalculated

Where Does Your SID Fall?

This gauge shows your current SID. Change values above to see the needle move.

Clinical Interpretation

SID Interpretation

What different SID values mean from the Stewart perspective.

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High SID (Alkalosis)

SID > 44 mEq/L
  • Chloride depletion (vomiting, NG suction)
  • Volume contraction
  • Excess sodium administration
  • Diuretic use
  • Post-hypercapnic alkalosis
  • Hypoalbuminemia (increases apparent SID effect)
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Normal SID

SID 36–44 mEq/L
  • Normal acid-base balance
  • Appropriate metabolic compensation
  • Balanced IV fluid administration
  • Normal renal function
  • Stable clinical state
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Low SID (Acidosis)

SID < 36 mEq/L
  • Hyperchloremic acidosis (NS overload)
  • Dilutional acidosis (excess free water)
  • Lactic acidosis
  • Renal tubular acidosis
  • Diarrhea
  • Unmeasured anion accumulation
Common Questions

Frequently Asked Questions

Answers to common questions about the Strong Ion Difference.

The SID is the net charge difference between strong (fully dissociated) cations and strong anions in the blood. SID = (Na⁺ + K⁺ + Ca²⁺ + Mg²⁺) − (Cl⁻ + Lactate). Normal SID is approximately 40 mEq/L (range 36–44). The SID is an independent variable in the Stewart approach that directly determines blood pH.
The traditional (Henderson-Hasselbalch) approach considers bicarbonate as an independent variable that you can directly manipulate. The Stewart approach recognizes that bicarbonate is a dependent variable determined by three independent factors: SID, total weak acid concentration (Atot), and pCO₂. This framework better explains complex ICU acid-base disorders and the effects of IV fluid administration on pH.
The normal apparent SID is approximately 40 mEq/L (range 36–44 mEq/L). This gap between strong cations and strong anions is filled by weak acid buffers — primarily bicarbonate (~24 mEq/L), albumin (~12 mEq/L), and phosphate (~2 mEq/L). When the SID changes, these weak acid buffers adjust, which changes pH.
Strong ions are completely dissociated at physiological pH. Strong cations: Na⁺, K⁺, Ca²⁺, Mg²⁺. Strong anions: Cl⁻, lactate, ketoacids, sulfate. Weak ions like bicarbonate, albumin, and phosphate are not strong ions — they exist in equilibrium between dissociated and undissociated forms and act as buffers.
Normal saline (0.9% NaCl) has equal concentrations of Na⁺ and Cl⁻ (154 mEq/L each), giving it an SID of zero. When you infuse NS, you add Na⁺ and Cl⁻ equally, but since plasma normally has a positive SID (~40), the NS dilutes this difference. The falling SID causes water to dissociate more, producing H⁺ and lowering pH. This is called dilutional acidosis or hyperchloremic acidosis.
Use the ion balance calculator in complex ICU settings where traditional acid-base analysis doesn't fully explain the clinical picture. It's particularly useful for evaluating the acid-base effects of IV fluids, understanding hyperchloremic acidosis, differentiating between types of metabolic acidosis, and assessing patients with multiple simultaneous acid-base disorders.