What is the osmolality of plasma if Serum sodium is 135 mEq/L, serum glucose is 120 mg/dL and BUN is 24 mg/dL?
## Core Concept
The osmolality of plasma is a measure of the concentration of osmotically active particles in blood plasma. It is primarily determined by the concentrations of sodium, glucose, and urea (BUN). The formula to calculate osmolality is: Osmolality (mOsm/kg) = (2 x Na+) + (Glucose / 18) + (BUN / 2.8), where sodium is in mEq/L, glucose in mg/dL, and BUN in mg/dL.
## Why the Correct Answer is Right
To calculate the osmolality of plasma, we substitute the given values into the formula: Osmolality = (2 x 135) + (120 / 18) + (24 / 2.8). Performing the arithmetic: Osmolality = 270 + 6.67 + 8.57 = 285.24 mOsm/kg. Rounding to the nearest whole number, we get approximately 285 mOsm/kg.
## Why Each Wrong Option is Incorrect
**Option A:** Without calculation, we cannot assume this value is correct or incorrect directly, but given the calculated value, we can assess its accuracy.
**Option B:** Similarly, without calculation, direct assessment is not possible, but if it does not match our calculated value, it is incorrect.
**Option C:** This option suggests an osmolality of 295 mOsm/kg, which seems slightly higher than what we calculated.
**Option D:** This option is not provided for evaluation.
## Clinical Pearl / High-Yield Fact
Normal plasma osmolality ranges from 275 to 295 mOsm/kg. An osmolality within this range indicates isotonic conditions, while values outside this range can indicate hypertonic (above 295) or hypotonic (below 275) conditions, which have significant clinical implications.
**Correct Answer:** . 285.