Reference values for osmolal gap in healthy subjects and in medical inpatients

Methanol and ethylene glycol poisonings are associated with high morbidity and mortality rates if treatment is not initiated early. Since few hospitals measure these toxic alcohols on a 24/7 basis, calculation of the osmolal gap (OG) is an important diagnostic tool. The reference value for the OG lacks consensus. We, therefore, wanted to update the reference value for OG in presumed healthy subjects and study OG values in internal medicine patients. The OG was calculated in 285 patients at the Medical Clinic at Oslo University Hospital, and in 118 healthy blood donors at Vestfold Hospital Trust. OG was calculated by the formula: OG = Measured osmolality - calculated osmolality ((1.86 × s-sodium + s-glucose + s-urea)/0.93) mOsm/kg H₂O. In the patients, median OG was 0 mOsm/kg H₂O (interquartile range -3 to 3 mOsm/kg H₂O, range -16 to103 mOsm/kg H₂O). When corrected for one outlier, the central 95% interval for OG was -10 to 20. The healthy blood donors had a median OG of -1 mOsm/kg H₂O (interquartile range -3 to1 mOsm/kg H₂O, range -13 to 8 mOsm/kg H₂O). When corrected for outliers, the reference range was -6 to 5 mOsm/kg H₂O. Based on results from a healthy population, we suggest a reference value for the OG of ≤5 mOsm/kg H₂O, but also recommend, based on our results from medical inpatients, to keep today's practice for suspecting poisoning with toxic alcohols at an elevated OG of ≥20 mOsm/kg H₂O.