Na in Posm formula

Q: While calculating Effective Plasma Osmolality, particularly in hyperosmotic hyperglycemic nonketotic state (HHNK), which of the sodium (Na) concentrations should be used? - select one

A) Actual measured plasma Na concentration

B) Corrected Na concentration

Answer: A

The formula for effective Plasma osmolality (Posm) is a useful method for diagnosing HHNK, where effective Posm is typically >320 mOsm/kg.

The formula is:

Effective Posm = [2 x Na (mEq/L)] + [glucose (mg/dL) ÷ 18]

The Na concentration used should be the actual measured plasma Na concentration, not the corrected Na concentration with high glucose.

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References:

1. Rasouli M. Basic concepts and practical equations on osmolality: Biochemical approach. Clin Biochem. 2016 Aug;49(12):936-41. doi: 10.1016/j.clinbiochem.2016.06.001. Epub 2016 Jun 22. PMID: 27343561.

2. Fazekas AS, Funk GC, Klobassa DS, Rüther H, Ziegler I, Zander R, Semmelrock HJ. Evaluation of 36 formulas for calculating plasma osmolality. Intensive Care Med. 2013 Feb;39(2):302-8. doi: 10.1007/s00134-012-2691-0. Epub 2012 Oct 19. PMID: 23081685.