Osmolarity of serum sodium in adults is?

AIIMS Rishikesh & Jodhpur NO - 2017

Biochemistry & Nutrition

Medium

Appeared in: AIIMS Rishikesh & Jodhpur NO - 2017

Explanation

The question asks for a value in mOsm/kg H₂O, which is the unit for osmolality, a measure of the total solute concentration in a fluid.
The normal range for serum osmolality in an adult is 285–295 mOsm/kg H₂O.
While serum sodium is the primary determinant of osmolality, the total value also includes contributions from glucose, urea, and other electrolytes.
The range 135-145 mEq/L refers to the serum sodium concentration, not the total serum osmolality.

Option A: This range (135-145) represents the normal serum sodium concentration, but the unit is incorrect. The correct unit for sodium concentration is mEq/L or mmol/L, not mOsm/kg H₂O.
Option B: This range (145-155 mOsm/kg H₂O) is above the normal range for serum osmolality and would indicate hyperosmolality. A serum sodium level above 145 mEq/L is defined as hypernatremia.
Option C: This range (165-175 mOsm/kg H₂O) is significantly outside the normal physiological range for either serum sodium concentration or total serum osmolality.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Normal laboratory values for serum osmolality in adults as background academic context rather than a clinical decision trigger.
Monitoring serum osmolality is crucial for assessing a patient's fluid and electrolyte balance, especially in conditions like dehydration, kidney disease, diabetes insipidus, or SIADH (Syndrome of Inappropriate Antidiuretic Hormone).
A nurse must understand the difference between serum sodium and serum osmolality to correctly interpret lab results and anticipate appropriate interventions, such as fluid administration or restriction.
What if? If a patient's lab report shows a serum sodium of 160 mEq/L, the nurse should anticipate that the serum osmolality will be high (hyperosmolar), likely well above 320 mOsm/kg H₂O, and would prepare for interventions to correct hypernatremia and dehydration.

How to Approach the Question

First, identify the core of the question: it asks for a specific laboratory value.
Pay close attention to the units provided in the options (mOsm/kg H₂O). This is a critical clue that the question is about osmolality, not just concentration.
Recall the normal ranges for common lab values. Differentiate between the normal range for serum sodium concentration (135-145 mEq/L) and total serum osmolality (285-295 mOsm/kg H₂O).
Analyze the options. Option A uses the numbers for sodium concentration but the units for osmolality, making it a tricky but incorrect distractor.
Select the option that provides the correct numerical range for the measurement indicated by the units (osmolality).

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