Enter the concentrations of sodium, potassium, calcium, magnesium, chloride, and lactate ions into the calculator to determine the apparent Strong Ion Difference (SIDa). Use the advanced tab to calculate SIDa, SIDe, and SIG using Stewart’s approach.
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Sid (Strong Ion Difference) Formula
The following formula is used to calculate the apparent SID (SIDa) value (a strong-ion sum, expressed in charge equivalents such as mEq/L):
SIDa = (Na⁺ + K⁺ + 2 × Ca²⁺ + 2 × Mg²⁺) − (Cl⁻ + Lactate⁻)
Variables:
- SIDa is the apparent Strong Ion Difference
- Na⁺ is the concentration of sodium ions
- K⁺ is the concentration of potassium ions
- Ca²⁺ is the concentration of calcium ions
- Mg²⁺ is the concentration of magnesium ions
- Cl⁻ is the concentration of chloride ions
- Lactate⁻ is the concentration of lactate ions
To calculate the SIDa value, add sodium and potassium, then add twice the calcium and magnesium concentrations (to account for their +2 charge). Subtract the sum of chloride and lactate from the previous result.
What is a Sid (Strong Ion Difference)?
The Strong Ion Difference (SID) is a concept used in acid–base physiology, particularly for understanding and managing acid–base disorders using Stewart’s approach. It is based on the principle that the pH of a solution (such as blood) is influenced by the difference between the concentrations of fully dissociated (or “strong”) cations and anions. In plasma, strong cations are primarily sodium and potassium (with smaller contributions from calcium and magnesium), and strong anions are primarily chloride and lactate. The SID (often calculated as the apparent SID, or SIDa) is a major determinant of bicarbonate concentration and thus pH. A higher SID tends to increase bicarbonate and raise pH (alkalosis), while a lower SID tends to decrease bicarbonate and lower pH (acidosis).
How to Calculate Sid (Strong Ion Difference)?
The following steps outline how to calculate the Sid (Strong Ion Difference).
- First, determine the concentration of strong cations in charge equivalents (mEq/L). If your calcium and magnesium are in mmol/L, multiply each by 2 to convert to mEq/L.
- Next, determine the concentration of strong anions in mEq/L.
- Next, subtract the concentration of strong anions from the concentration of strong cations.
- Finally, calculate the Sid (Strong Ion Difference).
- After inserting the variables and calculating the result, check your answer with the calculator above.
Example Problem :
Use the following variables as an example problem to test your knowledge.
concentration of strong cations (mEq/L) = 10
concentration of strong anions (mEq/L) = 5
