Unresponsive Patient With a Gap and a Gap
A 45-year-old man is brought in unresponsive. Glucose is 95 mg/dL, sodium 137 mmol/L, chloride 98 mmol/L, bicarbonate 6 mmol/L, BUN 14 mg/dL, creatinine 1.1 mg/dL, measured osmolality 350 mOsm/kg (freezing-point depression), ethanol 20 mg/dL, lactate 3.0 mmol/L, and arterial pH 7.18 with PCO2 18 mmHg. The chemistry analyzer reports sodium by indirect ion-selective electrode. Urinalysis shows a low specific gravity with numerous calcium oxalate monohydrate crystals. A friend reports the patient drank an unknown liquid at a party several hours earlier. The resident asks whether the lactate explains the acidosis and whether treatment can wait for a toxic alcohol level that will not return until morning.
1. Calculate the anion gap and the osmolal gap and interpret them together.
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Anion gap AG = Na − (Cl + HCO3) = 137 − (98 + 6) = 33 mmol/L (reference roughly 8–12). This is a marked high-anion-gap metabolic acidosis, consistent with the pH of 7.18 and appropriate respiratory compensation (PCO2 18).
Calculated osmolality 2(Na) + glucose/18 + BUN/2.8 + ethanol/4.6 = 274 + 5.3 + 5.0 + 4.3 = 288.6 mOsm/kg
Osmolal gap 350 − 288.6 = 61.4 mOsm/kg (normal generally <10–15).
Interpretation Both gaps are large. A high osmolal gap means an unmeasured, osmotically active solute is present; a simultaneous high anion gap means that solute is being metabolized to acid. That pairing is the signature of a toxic alcohol. Ethanol contributes only 4.3 mOsm/kg here and cannot account for the gap, and ethanol does not cause a high-anion-gap acidosis of this magnitude. Note that the osmolal gap falls as the parent alcohol is metabolized, so a normal gap later in the course does not exclude poisoning - the anion gap becomes the more useful marker.
2. Which toxin is most likely, and what should be done tonight while the level is pending?
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Most likely: ethylene glycol. Supporting features are the profound high-anion-gap acidosis, the large osmolal gap, and calcium oxalate monohydrate crystals in the urine. Methanol is the main alternative; isopropanol raises the osmolal gap but produces ketosis without a large anion gap.
Do not wait for the level. The specific level confirms but does not gate treatment; a delay allows metabolism to continue and worsens acidosis and organ injury.
Immediate steps
- Block alcohol dehydrogenase: fomepizole is first line. If unavailable, ethanol is an alternative but is harder to titrate and requires frequent levels.
- Bicarbonate for pH <7.2–7.3, targeting correction of the acidosis.
- Cofactors: thiamine and pyridoxine are given to shunt glyoxylate metabolism away from oxalate in ethylene glycol poisoning.
- Assess for hemodialysis: severe acidosis, renal failure, visual symptoms (methanol), or a very high level are indications. Dialysis removes both the parent alcohol and the toxic metabolites, and it is the definitive clearance mechanism.
- Supportive care: airway, thiamine and glucose if indicated, and serial electrolytes, blood gas, and osmolality.
Crystaluria supports the diagnosis, but its absence would not exclude ethylene glycol poisoning.
3. The resident argues the lactate of 3.0 mmol/L explains the anion gap, so no toxin workup is needed. Respond with the quantitative reasoning.
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Lactate is a minor contributor, not the explanation.
- Lactate at 3.0 mmol/L adds about 3 mmol/L to the anion gap (each mmol/L of unmeasured anion contributes roughly 1:1). The observed gap is 33, so lactate accounts for under 10% of it. A gap of this size requires a second organic acid - glycolate, oxalate, or formate.
- Lactate is also largely irrelevant to the osmolal gap: it is a charged species and does not behave as an unmeasured osmole. A modest lactate elevation does not raise the osmolal gap by 60 mOsm/kg.
- The mild lactate elevation is easily explained by hypoperfusion, agitation, or a seizure at presentation, and it does not require a toxin to be present - but it also cannot suppress the need to explain the other two findings.
The correct synthesis is a high-anion-gap metabolic acidosis plus a high osmolal gap, which requires a small uncharged molecule that is metabolized to acid. That reasoning supports empirical antidotal therapy now rather than a watch-and-wait approach. If the toxic alcohol level later returns undetectable, revisit the differential - but the treatment decision should not be deferred.