Anion Gap Theory
The anion gap estimates the difference between commonly measured positively charged ions and commonly measured negatively charged ions in plasma. An anion gap calculator helps identify whether extra unmeasured anions may be contributing to a metabolic acidosis pattern. The main value computed is the anion gap, with optional correction for albumin.
Core formulas
The standard teaching formula uses sodium, chloride, and bicarbonate:
\[
\begin{aligned}
AG &= [Na^+] - \left([Cl^-] + [HCO_3^-]\right)
\end{aligned}
\]
Some references include potassium:
\[
\begin{aligned}
AG &= [Na^+] + [K^+] - \left([Cl^-] + [HCO_3^-]\right)
\end{aligned}
\]
If albumin is low, the measured anion gap may underestimate the amount of unmeasured anions. A common correction is:
\[
\begin{aligned}
AG_{\text{corrected}} &= AG + 2.5 \cdot (4.0 - \text{albumin})
\end{aligned}
\]
In these equations, sodium, potassium, chloride, and bicarbonate are usually entered in mEq/L. Albumin is usually entered in g/dL. The correction assumes a normal albumin reference of 4.0 g/dL.
How to interpret the result
A normal anion gap means the measured cations and measured anions have the expected difference. If bicarbonate is low but the anion gap is normal, the pattern is often called normal-gap metabolic acidosis. A high anion gap suggests additional unmeasured anions, which is why the anion gap is central in metabolic acidosis analysis.
A corrected anion gap is especially useful when albumin is below normal because albumin is an important unmeasured anion. Low albumin can make the uncorrected anion gap look less abnormal than the acid-base disturbance really is.
Common pitfalls
- Mixing mmol/L and mEq/L without checking the calculator assumptions.
- Forgetting that low albumin can hide a high anion gap.
- Using the potassium-included formula while comparing to a range based on the standard formula.
- Interpreting the anion gap without checking pH, bicarbonate, and PCO₂.
Micro example
If sodium is 140 mEq/L, chloride is 100 mEq/L, and bicarbonate is 12 mEq/L:
\[
\begin{aligned}
AG &= 140 - (100 + 12) \\
&= 28\ \text{mEq/L}
\end{aligned}
\]
This is above the usual standard teaching range, so it suggests a high anion gap pattern.
When to use it
This tool is useful when reviewing electrolyte panels, metabolic acidosis patterns, or the effect of albumin correction on acid-base interpretation. It should not be used alone as a diagnosis; the next step is to interpret the result together with pH, PCO₂, bicarbonate, clinical context, and compensation rules.
