Formate kinetics in methanol poisoning

The objective is to describe the kinetics of formate, the main toxic metabolite of methanol, in a series of consecutive patients treated in the same intensive care unit for severe methanol poisoning.

The charts of the patients admitted between 1987 and 2001 were reviewed. Inclusion criteria were: a history of deliberate methanol ingestion, with a blood methanol concentration greater than 20 mg/dL (6.2 mmol/L) or a high anion gap metabolic acidosis. Indications for hemodialysis were: blood methanol concentration >50 mg/dL (15.8 mmol/L), metabolic acidosis (bicarbonate <15 mmol/L, arterial pH <7.30), visual toxicity. Antidotal therapy included ethanol administration in 22 cases, and fomepizole in three cases. Serial blood measurements were obtained for pH, bicarbonate, methanol and formate. Endogenous and hemodialysis elimination half-lives were calculated as t1/2 =0.693/Ke. Fick principle was applied for hemodialysis clearance calculation.

The records of 25 methanol poisoned patients were analysed. Among them, 18 patients had sufficient data to allow accurate determinations of formate kinetics. Formate half-life elimination during hemodialysis was 1.80+/-0.78 h, which was statistically different from the values observed before or in the absence of dialysis (6.04+/-3.26 h, P =0.004). The mean hemodialysis formate clearance rate calculated in eight cases was 176+/-43 mL/min. A rebound in plasma formate concentration was observed in three patients after the discontinuation of hemodialysis.

In accordance with previous isolated case reports and in contrast with a recent case series, our data document that hemodiaysis is effective in reducing formate elimination half-life. The impact on clinical outcome is still debatable.