SID (Strong Ion Difference) Calculator

Formula

The formula to calculate the Strong Ion Difference (SID) is:

\[ SID = (Na^+ + K^+ + 2 \times Ca^{2+} + 2 \times Mg^{2+}) - (Cl^- + Lactate^- + 2 \times HCO_3^-) \]

Where:

\(SID\) is the Strong Ion Difference
\(Na^+\) is the concentration of sodium ions
\(K^+\) is the concentration of potassium ions
\(Ca^{2+}\) is the concentration of calcium ions
\(Mg^{2+}\) is the concentration of magnesium ions
\(Cl^-\) is the concentration of chloride ions
\(Lactate^-\) is the concentration of lactate ions
\(HCO_3^-\) is the concentration of bicarbonate ions

What is SID (Strong Ion Difference)?

The Strong Ion Difference (SID) is a concept used in the field of acid-base physiology, particularly in the context of understanding and managing acid-base disorders. It is based on the principle that the pH of a solution (such as blood) is determined by the difference between the concentrations of fully dissociated (or “strong”) cations and anions. In other words, it is the difference between the sum of the concentrations of the strong cations (mainly sodium, potassium, and calcium) and the strong anions (mainly chloride and lactate) in plasma. The SID is a measure of the net electrical charge of the strong ions in solution and is a major determinant of the bicarbonate concentration and thus the pH. A high SID (more cations than anions) will increase bicarbonate concentration and raise pH (alkalosis), while a low SID (more anions than cations) will decrease bicarbonate concentration and lower pH (acidosis).

Example Calculation

Let's assume the following values:

Concentration of Sodium Ions (Na⁺) = 140 mmol/L
Concentration of Potassium Ions (K⁺) = 4 mmol/L
Concentration of Calcium Ions (Ca²⁺) = 2.5 mmol/L
Concentration of Magnesium Ions (Mg²⁺) = 1.5 mmol/L
Concentration of Chloride Ions (Cl^-) = 100 mmol/L
Concentration of Lactate Ions (Lactate^-) = 1.2 mmol/L
Concentration of Bicarbonate Ions (HCO₃^-) = 24 mmol/L

Using the formula to calculate the Strong Ion Difference (SID):

\[ SID = (140 + 4 + 2 \times 2.5 + 2 \times 1.5) - (100 + 1.2 + 2 \times 24) = 152.0 - 149.2 = 2.8 \text{ mmol/L} \]

The Strong Ion Difference (SID) is approximately 2.8 mmol/L.