Enter all but one of the concentrations of sodium, potassium, calcium, magnesium, chloride, lactate, and bicarbonate ions into the calculator to determine the Strong Ion Difference (Sid); this calculator can also evaluate any of the variables given the others are known.

Sid (Strong Ion Difference) Formula

The following formula is used to calculate the Sid (Strong Ion Difference) value:

Sid = (Na+ + K+ + 2 * Ca2+ + 2 * Mg2+) – (Cl- + Lactate- + 2 * HCO3-)

Variables:

  • Sid is the 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
  • HCO3- is the concentration of bicarbonate ions

To calculate the Sid value, sum the concentrations of sodium, potassium, twice the concentrations of calcium and magnesium ions. Subtract the sum of the concentrations of chloride, lactate, and twice the concentration of bicarbonate ions from the previous result.

What is a 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).

How to Calculate Sid (Strong Ion Difference)?

The following steps outline how to calculate the Sid (Strong Ion Difference).

  1. First, determine the concentration of strong cations (mEq/L).
  2. Next, determine the concentration of strong anions (mEq/L).
  3. Next, subtract the concentration of strong anions from the concentration of strong cations.
  4. Finally, calculate the Sid (Strong Ion Difference).
  5. 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