Hyponatremia vs. Pseudohyponatremia

Case scenario: I had a patient with MG crisis and had hyponatremia after IVIG (Na in BMP dropped from 134 to 124 on Day 4), checked at that same time serum osmolality was 269, calculated serum osmolality was 259. So is it pseudohyponatremia? What workup do I need to to differentiate pseudohyponatremia and true hyponatremia?

  1. Check ABG or VBG, the blood gas analyzer is a direct ion-specific electrode (ISE) allows measurement of plasma sodium directly from a whole-blood sample. 
  2. Calculated Serum osmolality vs measured serum osmolality, the higher the gap the more likely it is pseudohyponatremia.
  3. Calculate water content of the serum. check lipid panel and LFT for total protein Reduced % water content (<93 %) indicates pseudohyponatremia. Plasma water % = 99.1 – (1.03 × lipid concentration g/L) – (0.73 × protein concentration g/L)

Further reading:

This article explains the concept really well – why pseudohyponatremia exists? It usually depends on the lab technique they used to measure Na. https://acutecaretesting.org/en/articles/pseudohyponatremia

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