Explanation of Units
Since laboratories measure serum solutes in terms of freezing point depression, the reported units are properly units of osmolality. When a measure of serum solutes is calculated, it is often done in units of osmolarity. While it is possible to convert between osmolality and osmolarity, thereby deriving a more mathematically correct osmol gap calculation, in actual clinical practice this is not done. This is because the difference in absolute value of these two measurements that can be attributed to the difference in units will be negligible in a clinical setting. For this reason, the terms are often used interchangeably, though some object to equating the terms. Because the calculated osmol gap can therefore be a conflation of both terms (depending on how it is derived), neither term (osmolal gap nor osmolar gap) may be semantically correct. To avoid ambiguity, the terms "osmolal" and "osmolar" can be used when the units of molality or molarity are consistent throughout the calculation. When this is not the case, the term "osmol gap" can be used when units are mixed to provide a clinical estimate.
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