From the Guidelines
The patient has a significant osmolar gap, indicating the presence of unmeasured osmotically active substances, which could be due to ethanol or other toxic alcohols like methanol or ethylene glycol. To calculate the osmolar gap, we use the formula: 2(Na) + Glucose/18 + BUN/2.8, which gives us approximately 278 mOsm/kg (2×134 + 88/18 + 22/2.8), while the measured serum osmolality is 332 mOsm/kg, resulting in an osmolar gap of about 54 mOsm/kg. The normal range for the osmolar gap is less than 10-15 mOsm/kg 1. The elevated osmolar gap in this patient is consistent with the presence of unmeasured osmotically active substances. The ethanol level of 134 mg/dL contributes approximately 29 mOsm/kg to serum osmolality, leaving a remaining gap that could suggest the presence of other toxic alcohols. According to the EXTRIP workgroup, an osmol gap > 50 is a recommendation for ECTR if ethanol is used as an antidote 1. Given the patient's elevated osmolar gap and ethanol level, close monitoring, supportive care, and possibly specific treatments like fomepizole or hemodialysis are necessary, depending on the clinical presentation and suspicion for ingestion of other alcohols. It is essential to consider the potential risks and benefits of each treatment option, including the costs and availability of ECTR and antidotes like fomepizole, as well as the patient's individual clinical scenario 1. The decision to initiate ECTR should be based on the patient's overall clinical condition, including the presence of coma, seizures, or kidney impairment, as well as the EG concentration and osmol gap 1.
From the Research
Calculating Osmolal Gap
To determine if there is an osmolar gap, we need to calculate the estimated serum osmolality using the formula:
- Estimated serum osmolality = (1.86 × sodium) + (glucose / 18) + (BUN / 2.8)
- Then, subtract the estimated serum osmolality from the measured serum osmolality to find the osmolal gap.
Given Lab Values
- Sodium: 134 mmol/L
- BUN: 22 mg/dL
- Glucose: 88 mg/dL
- Ethanol level: 134 mg/dL
- Serum osm: 332 mOsm/kg H2O
Calculation
- Estimated serum osmolality = (1.86 × 134) + (88 / 18) + (22 / 2.8)
- Estimated serum osmolality = 249.24 + 4.89 + 7.86
- Estimated serum osmolality = 261.99 mOsm/kg H2O
- Osmolal gap = Measured osmolality - Estimated osmolality
- Osmolal gap = 332 - 261.99
- Osmolal gap = 70.01 mOsm/kg H2O
Interpretation
- According to the study 2, a reference value for the osmolal gap of ≤5 mOsm/kg H2O is suggested for healthy individuals, but for medical inpatients, an elevated osmolal gap of ≥20 mOsm/kg H2O is recommended for suspecting poisoning with toxic alcohols.
- The calculated osmolal gap of 70.01 mOsm/kg H2O is significantly higher than the recommended threshold, indicating the presence of an osmolar gap.
- The presence of ethanol in the patient's blood, as reported in the study 3, can contribute to the elevated osmolal gap.
- Other studies 4, 5, 6 also discuss the importance of considering the osmolal gap in the diagnosis of toxic alcohol ingestions, such as methanol and ethylene glycol poisoning.