The plasma osmolality is calculated using the formula?

CGHS SSC - 2018

Edition 3

Easy

Appeared in: CGHS SSC - 2018

Explanation

This option is the most accurate choice as it correctly identifies the three main solutes that determine plasma osmolality: sodium, blood urea nitrogen (BUN), and glucose.
Sodium (Na+) is the most abundant cation in the extracellular fluid, and its concentration is doubled in the formula to account for the associated anions (like chloride and bicarbonate).
The formula correctly includes terms for glucose and BUN, which are the other major osmotically active substances in the plasma.
While the divisors for BUN and glucose in the option are approximations (especially for glucose), the overall structure of the formula is the most correct among the choices provided.

Option A: This formula is incorrect because it only considers bicarbonate. While bicarbonate is an electrolyte, it is not the primary determinant of osmolality, and its contribution is already factored into the doubling of sodium in the correct formula.
Option B: This formula is incorrect. It uses chloride instead of sodium as the primary electrolyte and uses an arbitrary multiplier of 3. Sodium is the main cation influencing osmolality.
Option D: This formula is incorrect because it uses chloride instead of sodium and completely omits the contribution of glucose, which can significantly impact osmolality, especially in conditions like diabetes.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Calculation of Plasma Osmolality as background academic context rather than a clinical decision trigger.
Calculating plasma osmolality is crucial for determining the osmolal gap (Measured - Calculated Osmolality), which helps in the differential diagnosis of metabolic acidosis and suspected toxic ingestions (e.g., methanol, ethylene glycol).
A normal osmolal gap is less than 10 mOsm/kg. A value greater than this suggests the presence of unmeasured osmoles in the blood, requiring urgent investigation.
This calculation is essential for classifying hyponatremia (low sodium) into hypotonic, isotonic, or hypertonic, which guides the appropriate treatment strategy.

How to Approach the Question

First, recall the main substances that contribute to the osmotic pressure of plasma. These are primarily electrolytes (sodium and its accompanying anions) and major non-electrolytes (glucose and urea).
Analyze the lead-in, which asks for the 'formula'. This is a direct recall question.
Evaluate each option based on the key components. Look for the formula that includes sodium (Na+), glucose, and urea (BUN).
Recognize that the sodium concentration is typically doubled (2 × Na+) to account for the anions that balance its positive charge.
Select the option that best incorporates these three components (Na+, Glucose, BUN) in their correct relationship, even if the specific numbers are approximations.

Concept Tested & Keywords

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