Nephrology MOC Sample Questions

The correct answer is B. Asking the chemistry lab to measure serum sodium concentration using direct potentiometry

Explanation: The normal serum osmolality rules out hyp-osmolal hyponatremia and thus disorders such as SIADH. It also rules out hyper-osmolal hyponatremia such as occurs with hyperglycemia, thus making psuedohyponatremia the most likely diagnosis. Psuedohyponatremia was observed at first when chemistry labs used flame photometry to measure serum sodium concentration. Flame photometry measures the sodium concentration in the entire volume of serum not just the aqueous portion. Nowadays flame photometry is no longer used to measure serum sodium concentration. Potentiometry measures serum sodium concentration in the aqueous phase of serum and thus psuedohyponatremia should no longer be observed. Except for the fact that many labs in the United States dilute the serum sample before measuring the serum sodium concentration (indirect potentiometry) and as such we still do see psuedohyponatremia as shown in the figure below. Measuring the sodium concentration without first diluting the serum sample should result in normal value for serum sodium concentration (direct potentiometry).