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Estimating Plasma Levels of Methanol and Formic Acid from Chemistry Tests

Purpose: To use the osmolal gap and arterial pH to evaluate a patient with methanol poisoning.

Description

Methanol is metabolized by alcohol dehydrogenase to formaldehyde, which is then metabolized to formic acid. This results in a metabolic acidosis and increase in the osmolal gap. The osmolal gap can be used to estimate the plasma methanol level, while the arterial pH correlates with the formic acid concentration.


According to Cadnapaphornchai et al (Figure 4, page 96):

 

osmolal gap in mOsm/kg water =

= (0.29 * (methanol plasma level in mg/dL)) + 4.5

 

Rearranged:

 

estimated methanol plasma level in mg/dL =

= ((osmolal gap in mOsm/kg water) – 4.5) / 0.29 =

= (3.45 * (osmolal gap in mOsm/kg water) – 15.52

 

The line in Figure 2 from Brent et al (page 427) for correlating the arterial pH and plasma formic acid concentration can be approximated as:

 

predicted arterial pH =

= 7.42437 – (0.01403 * (plasma formic acid concentration in mmol/L))

 

Rearranged:

 

estimated formic acid concentration in mmol/L =

= ((arterial pH) – 7.42437) / (-0.01403)


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Specialty

Objective

ICD-10