NEET 2014 Surgery MCQ

A postoperative patient has a serum sodium concentration of 125 mEq/L and a blood glucose level of 500 mg/dL (normal level, 100 mg/dL). What would the patient’s serum sodium concentration be (assuming normal renal function and appropriate intraoperative fluid therapy) if the blood glucose level were normal?

A 120 mEq/L
B 122 mEq/L
C 137 mEq/L
D 142 mEq/L
E 147 mEq/l

Explanation -
Serum osmolality is described as the amount of solutes per unit of water. It can be measured with an osmometer or it can be calculated. It is reported as milliosmoles per liter. The serum concentrations of sodium, urea, and glucose are required, whereas
that of chloride is not required for the calculation. Simply doubling the serum sodium concentration provides an adequate estimate of serum osmolality. As a general rule, each 100-mg/dL rise
in the blood glucose level above normal is equivalent to a 1.6- to 3.0-mEq/L fall in the apparent serum sodium concentration. For example, if the patient has a blood glucose level of 500 mg/dL,
or 400 mg/dL above normal, this is equivalent to a 12-mEq/L change in the serum sodium level. If this patient has a measured sodium concentration of 125 mEq/L, the sodium concentration is
actually 137 mEq/L once the excess extracellular water has been eliminated