What is the most accurate formula for predicting plasma osmolality?

The most accurate formula for predicting plasma osmolality is the one that considers the primary osmotic contributors in the plasma and uses appropriate units for their concentrations. In this case, the formula involving sodium, glucose, and BUN (blood urea nitrogen) provides a precise estimation of plasma osmolality.

Sodium is the chief cation in the extracellular fluid and plays a critical role in regulating osmotic balance. To account for the osmolality contributed by glucose and urea, this formula adjusts their concentrations by using the factors of 18 and 2.8, respectively. These adjustments convert the glucose and BUN concentrations from mg/dL to mmol/L, which are the standard units used when calculating osmolality.

The other options do not provide the same level of accuracy as they either fail to include all relevant solutes or do not convert solute measurements accurately into effective osmoles. Therefore, the selected formula stands out because it integrates the essential contributors with appropriate unit conversions, making it the most precise method for estimating plasma osmolality.