In a patient who has severe alcohol intoxication, the osmolal gap can be used to estimate the serum alcohol level. This is based on alcohol serving as an osmotically active substance.
estimated blood alcohol (mg/dL) =
= (osmolal gap in mOsm/kg water) * 4.54
where:
• The osmolal gap is calculated as shown in the chapter on endocrinology and metabolism (Chapter 13).
• The serum osmolality increases 22 mOsm/kg for every 100 mg/dL of ethanol; 4.54 is 100/22.
In SI units:
estimated blood alcohol (mmol/L) = osmolal gap in mOsm/kg water
where:
• The conversion factor from conventional to SI units is 0.217 for mg/dL to mmol/L.
• 4.54 * 0.217 = 0.99
Purssell et al developed equations using linear regression to predict the osmolal gap from the serum ethanol level:
osmolal gap in mOsm/kg water =
= ((serum ethanol in mg/dL) / 3.7) – 0.35 =
= (0.2702 * (serum ethanol in mg/dL) – 0.35 =
= (1.25 * (serum ethanol in mmol/L)) – 0.35
Rearranged:
serum ethanol in mg/dL =
= (3.7 * ((osmolal gap in mOsm/kg water) + 0.35) =
serum ethanol in mmol/L =
= (0.8 * ((osmolal gap in mOsm/kg water) + 0.35)
Cadnapaphornchai et al (Figure 4, page 96) plotted the plasma level of ethanol vs the osmolal gap in mOsm/kg water:
osmolal gap in mOsm/kg water =
= (0.21 * (plasma level of ethanol in mg/dL)) + 0.55
Rearranged:
serum ethanol in mg/dL =
= 4.76 * ((osmolal gap in mOsm/kg water) – 0.55) =
= 4.76 * ((osmolal gap in mOsm/kg water) – 2.62
Limitations:
• The assumption is that any difference in measured from calculated osmolality is due solely to ethanol. The presence of other osmotically active substances or underlying disease could also be present, causing a falsely high ethanol estimate.
