Effective Osmolarity Calculator

Formula

The formula to calculate the Effective Osmolarity (O) is:

$O = 2 \cdot Na + \left( \frac{G}{18} \right)$

Where:

$O$ is the effective osmolarity (mOsm/L)
$Na$ is the serum sodium concentration (mEq/L)
$G$ is the glucose concentration (mg/dL)

What is Effective Osmolarity?

Effective osmolarity, also known as tonicity, is a measure of the osmotic pressure exerted by solutes in a solution that contribute to the movement of water across cell membranes. It is an important parameter in clinical settings to assess the balance of fluids and electrolytes in the body. Effective osmolarity takes into account only those solutes that do not freely cross cell membranes, such as sodium and glucose, and thus directly influence the distribution of water between intracellular and extracellular compartments.

Example Calculation

Let's assume the following values:

Serum Sodium Concentration (Na) = 140 mEq/L
Glucose Concentration (G) = 90 mg/dL

Using the formula to calculate the Effective Osmolarity:

$O = 2 \cdot 140 + \left( \frac{90}{18} \right) = 280 + 5 = 285 \text{ mOsm/L}$

The Effective Osmolarity is 285 mOsm/L.