American Board of Internal Medicine (ABIM) Certification Practice Exam

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Which calculation is used for serum osmolality?

2Na + BUN/1.5 + glucose/12
2Na + BUN/4 + glucose/20
2Na + BUN/2.8 + glucose/18
2Na + BUN/3 + glucose/15

The correct answer is: 2Na + BUN/2.8 + glucose/18

The calculation for serum osmolality is derived from the concentrations of sodium, blood urea nitrogen (BUN), and glucose in the blood. The most widely accepted formula is: Serum Osmolality = 2(Na) + (BUN/2.8) + (glucose/18) Using this equation, the sodium concentration is multiplied by 2, reflecting the contribution of sodium ions to osmolality, which is significant given that sodium is a major extracellular cation. The BUN value is then divided by 2.8 to account for its contribution to osmotic pressure, and glucose is divided by 18, appropriately reflecting its impact on serum osmolality. In this context, the correct answer includes the appropriate divisions for both BUN and glucose, making it the accurate option for calculating serum osmolality. Understanding the specific values for BUN and glucose is essential to obtaining an accurate osmolality measurement as both substances contribute to the overall osmotic pressure in the bloodstream.