How to Calculate Osmolality

Osmolality is the equation used by chemists to calculate the number of particles in a solution. While molarity is a count of particles in a solution of a specific chemical, osmolality is the whole number of particles in the entire fluid. Osmolality is used in medicine mostly for calculations in renal abnormalities. Since the kidneys filter many of the ions in the body, calculating osmolality helps doctors evaluate the current efficiency of the renal system. Normal serum osmolality levels are 285-295 mOsm/L.

Instructions

• 1

  Find the glucose serum levels. Part of the equation for osmolality is glucose levels divided 18. For example, use 100 mg/dL. 100/18= 5.56 mg/dL.

• 2

  Find serum sodium (Na) levels. For this example, use 135 mEq/L.

• 3

  Find BUN serum levels. BUN levels are the byproduct of protein waste filtered by the kidneys. For this example, use 1.4 mg/dl. BUN levels in osmolality calculations are divided by 2.8.

• 4

  Setup the equation. For this example, the equation is setup as:

  Osmolality = 2 * 135 + 100/18 + 1.4/2.8

• 5

  Calculate the final osmolality.

  Osmolality = 270 + 5.56 + .5 = 276.06