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The Difference Between Actual and Calculated Osmolality of IU Solutions Should Not Be Overlooked

Swank, Kenneth M. MD

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doi: 10.1213/00000539-199912000-00071
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I read with interest the article by Williams et al. (1) discussing the effects of lactated Ringer’s solution and normal saline on serum osmolality. In the introduction to the article, the authors state that the “measured osmolality of normal saline is similar to the calculated osmolarity of 308 mOsm/L.” However, according to Weast (2), the osmolarity of a 0.9% sodium chloride solution is actually 287 mOsm/kg. How do the authors explain this discrepancy? A smaller difference in osmolality between the two solutions (33 mOsm/kg) instead of their larger assumed difference 54 mOsm/kg) may explain why the difference in obtained serum osmolality between the two solutions was small.

Kenneth M. Swank MD


1. Williams E, Hildebrand KL, McCormick SA, et al. The effect of intravenous lactated ringer’s solution versus 0.9% sodium chloride on serum osmolality in human volunteers. Anesth Analg 1999; 88: 999–1003.
2. Weast RC, ed. CRC handbook of chemistry and physics. 68th ed. CRC Press, 1987:D-253, Table 71.
© 1999 International Anesthesia Research Society