Potassium balance model
A potassium balance model explains why the measured serum potassium value may not fully represent total body potassium. Most potassium is inside cells, so serum K⁺ can change quickly when potassium shifts between the intracellular and extracellular compartments.
This calculator estimates whether the pattern suggests a potassium deficit, excess tendency, or mainly redistribution. It combines measured serum K⁺, body weight, acid-base influence, and teaching presets such as insulin, beta-adrenergic stimulation, cell release, or renal retention.
Core definitions and formulas
The first step is to estimate the total shift effect on the measured serum potassium:
\[
\begin{aligned}
\Delta K_{\text{shift}}
&= \Delta K_{\text{acid-base}} + \Delta K_{\text{preset}}
\end{aligned}
\]
A positive shift effect means K⁺ is moving out of cells and raising serum potassium. A negative shift effect means K⁺ is moving into cells and lowering serum potassium.
The calculator then estimates an adjusted potassium value by removing the selected shift effect from the measured value:
\[
\begin{aligned}
K_{\text{adjusted}}
&= K_{\text{measured}} - \Delta K_{\text{shift}}
\end{aligned}
\]
For a simple teaching estimate, deficit or excess tendency is scaled by body weight. The calculator uses a 70 kg reference body weight and estimates about 100 mEq for each 0.30 mmol/L outside the selected potassium reference range.
\[
\begin{aligned}
\text{Weight scale}
&= \frac{\text{body weight}}{70}
\end{aligned}
\]
How to interpret results
A low measured serum K⁺ may reflect true whole-body potassium deficit, intracellular shifting, or both. A high measured serum K⁺ may reflect potassium excess, impaired renal handling, acidosis-related extracellular shift, cell release, or a combination of processes.
The most important comparison is between measured serum potassium and the shift-adjusted teaching estimate. If these differ strongly, redistribution is likely contributing to the measured value. If the adjusted value remains outside the reference range, the model suggests a stronger deficit or excess tendency.
- Do not treat the mEq estimate as a direct replacement dose.
- Check whether the abnormal value could be caused by transcellular shift.
- Use body weight in kilograms.
- Remember that renal handling strongly affects potassium balance.
Micro example
If measured K⁺ is \(3.0\ \text{mmol/L}\), alkalosis contributes \(-0.60\ \text{mmol/L}\), and insulin contributes \(-0.60\ \text{mmol/L}\), then:
\[
\begin{aligned}
\Delta K_{\text{shift}}
&= -0.60 + (-0.60) \\
&= -1.20\ \text{mmol/L}
\end{aligned}
\]
The adjusted estimate is:
\[
\begin{aligned}
K_{\text{adjusted}}
&= 3.0 - (-1.20) \\
&= 4.20\ \text{mmol/L}
\end{aligned}
\]
This pattern teaches that the low measured serum K⁺ may be largely explained by intracellular redistribution rather than a simple whole-body deficit alone. This tool is best used for physiology learning, acid-base interpretation, and potassium distribution reasoning, while more advanced analysis should include kidney function, intake, losses, medications, and clinical context.
