Osmolarity is a measure of the concentration of the solutes in a solution, and is measured in moles of solute particles in a given volume of solution. Plasma osmolarity refers specifically to the osmolarity of blood plasma and typically measures only specific solutes. It is a common diagnostic tool for identifying electrolyte imbalances, especially low blood sodium (hyponatremia). Plasma osmolarity may be calculated from the concentrations of the individual solutes.

Obtain the concentrations for the solutes you wish to calculate the plasma osmolarity for. The most common solutes of interest include sodium (Na+), glucose and blood urea nitrogen (BUN).

Convert mg/dl to the standard units of millimoles per liter (mmol/L). To convert deciliters to liters, multiply by 10. To convert milligrams to millimoles, divide by the molecular weight. Therefore, to convert mg/dl to mmol/L, multiply by 10/Ds, where Ds is the molecular weight of the solute s.

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Calculate 10/Ds where Ds is the molecular weight for glucose and BUN. The molecular weight for glucose is 180, so 10/Ds for glucose is 1/18. BUN is diatomic nitrogen (N2), so its molecular weight is 28. Therefore, 10/Ds for BUN is 10/28 = 1/2.8. We now have a plasma molarity of [Na+] + [Glucose]/18 + [BUN]/2.8, where [Na+], [Glucose] and [BUN] indicate the respective concentrations of these solutes in standard units of mmol/L.

Calculate the plasma osmolarity from the plasma molarity obtained in Step 3, [Na+] + [Glucose]/18 + [BUN]/2.8. To convert to osmolarity, multiply the molarity values by the number of particles each of these solutes dissociates into. Na+ dissociates into two particles, while glucose and BUN each dissociate into one particle. Thus, the osmolarity is 2[Na+] + [Glucose]/18 + [BUN]/2.8.

Use standard reference values for the calculation of plasma osmolarity. The standard Na+ concentration is 140 millimoles/liter (mmol/L), the standard glucose concentration is 150 milligrams/deciliter (mg/dl) and the standard BUN concentration is 20 mg/dl. From the equation 2[Na+] + [Glucose]/18 + [BUN]/2.8 in Step 4, we have 2(140) + (150/18) + (20/2.8) = 280 + 8.3 + 7.1 = 295. A typical plasma osmolarity is therefore about 295 mmol/L.