Serum osmolarity can be calculated using the following formula:
Serum osmolarity = 2(sodium) + glucose/18 + BUN/
2.8 + ethanol/4.6 + mannitol/18
This patients calculated osmolarity is approximately 300 mOsm/L. The patients osmolar gap can be obtained by subtracting the calculated serum osmolarity from the measured serum osmolarity (352-
300 = 52). Thus, the patient has an osmolar gap of 52 mOsm/L. An osmolar gap occurs when a solute other than sodium, glucose, or urea is added to the plasma. When another solute is present, the osmolarity calculated from the above formula will be less than the actual measured value, causing an osmolar gap. An osmolar gap of 25 mOsm/L or greater is highly suggestive of ethylene glycol or methanol intoxication. Both of these intoxications should be entertained as etiologies of this patients symptoms.
- 52 mOsm/L.
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