OG Om Oc

Normally, the osmolality of serum is determined by sodium (and its accompanying anions), glucose, and urea according to the formula (SI units)

Oc = (1.86[Na] + [urea] + [glucose])/0.93. The reference range for the osmolal gap is 5 ± 7 mosmol/kgH2O.

The intoxicants best able to increase the osmolal gap are those which have a low molecular weight and are present in high mass units, i.e. high molar concentrations. The lower alcohols and glycols are such substances. A methanol concentration of 32 mmol/l (1 g/l) increases the osmolal gap by 32/0.93 = 34 mosmol/kgH 2O. The osmolal contribution of methanol is so significant that interference from other causes will only occur at methanol levels below 16 mmol/l (0.5 g/l).

Only methanol and ethylene glycol regularly cause severe metabolic acidosis and elevation of both the anion and osmolal gaps. However, if ethanol is coingested with methanol or ethylene glycol, there will be no metabolic acidosis until most of the ethanol is metabolized owing to its antidotal effect. In such circumstances calculation of the gaps must be repeated.

In the late stages of methanol or ethylene glycol poisoning, most of the alcohol or glycol may be metabolized to its acidic metabolite. In this situation there is a pronounced metabolic acidosis with a high anion gap. Owing to low alcohol/glycol levels, the osmolal gap may be close to normal values, particularly in ethylene glycol poisoning because of its higher molecular weight and thus smaller molar contribution. In this situation, a small or normal osmolal gap does not eliminate the possibility of toxic alcohol ingestion.

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