Anion and osmolal gaps in the diagnosis of methanol poisoning: clinical study in 28 patients

Objective: To evaluate anion and osmolal gaps as diagnostic tools in methanol poisoning. Design and setting: Clinical observational study. Patients and methods: In a recent methanol outbreak, the initial triage and treatment decisions in 28 patients were based mainly upon the values of the osmolal and anion gaps on admission. Methanol and formate levels were later compared to these gaps by linear regression analysis. Results: The correlation between the osmolal gaps and serum methanol concentrations on admission was linear (y=1.03x+12.71, R-2=0.94). The anion gaps correlated well with the serum formate concentrations (y=1.12x+13.82, R-2=0.86). Both gaps were elevated in 24 of the 28 subjects upon admission. Three patients had an osmolal gap within the reference area (because of low serum methanol), but elevated anion gap because of formate accumulation. One patient with probable concomitant ethanol ingestion had a high osmolal gap and a normal anion gap. Conclusion: Osmolal and anion gaps are useful in the diagnosis and triage of methanol-exposed subjects. Confounders are low serum methanol and concomitant ethanol ingestion.