Hovda et al used the anion and osmolal gaps to guide management of a patient with methanol poisoning. The authors are from Ullevaal University Hospital and Ostfold Central Hospital in Norway.
Molecular weight of methanol: 32.04 g
Molecular weight of formic acid: 46.03 g
Parameters:
(1) anion gap in mmol/L
(2) osmolal gap in mOsm per kg water
Anion Gap
Osmolal Gap
Status
Management Steps
<= 20
<= 25
subtoxic
A, B
<= 20
> 25
early toxic
B, C, D, F
> 20
<= 25
late toxic
B, C, D, E, F (if visual disturbances present)
> 20
> 25
intermediate toxic
B, C, D, E, F
A: Observe
B: Monitor blood gases.
C: Administer bicarbonate if in metabolic acidosis
D: Administer antidote (fomepizole)
E: Administer folinic acid.
F: Consider hemodialysis
The authors also correlated serum methanol and folate levels with osmolal and anion gaps, respectively.
osmolal gap in mOsm per kg water =
= (1.03 * (serum methanol in mmol/)) + 12.71
If this is converted to mg/dL
osmolal gap in mOsm per kg water =
= (0.3215 * (serum methanol in mg/dL)) + 12.71
anion gap in mmol/L =
= (1.12 * (serum formate in mmol/L)) + 13.82
If this is converted to mg/dL
anion gap in mmol/L =
= (0.2433 * (serum formate in mg/dL)) + 13.82
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