Urine Findings in Antifreeze (Ethylene Glycol) Poisoning
The most characteristic urine finding in antifreeze poisoning is the presence of calcium oxalate crystals, which appear as envelope-shaped monohydrate or dumbbell-shaped dihydrate crystals, indicating metabolism of ethylene glycol to its toxic metabolites. 1
Key Urinary Indicators
Calcium Oxalate Crystals
- Calcium oxalate crystals in urine are pathognomonic for ethylene glycol poisoning and indicate that toxic metabolism has already occurred 2, 1
- These crystals form when oxalic acid (the end metabolite of ethylene glycol) precipitates with calcium in the renal tubules 2, 3
- The presence of oxalate crystals should trigger immediate treatment with fomepizole and consideration for hemodialysis 1
- Crystal deposition in renal tubules causes acute tubular necrosis and acute kidney injury 2, 3
Calcium Oxaluria
- Calcium oxaluria (oxalate in urine) develops as ethylene glycol is metabolized through alcohol dehydrogenase to glycolic acid, glyoxylic acid, and finally oxalic acid 1
- This finding indicates advanced poisoning and significant risk for permanent renal damage 3
Hematuria
- Microscopic or gross hematuria may be present due to tubular injury from crystal deposition 4
- Hematuria combined with acute kidney injury should raise suspicion for ethylene glycol poisoning in the appropriate clinical context 4
High Urine Ethylene Glycol and Glycolate Concentrations
- In fatal ethylene glycol poisonings, median urine concentrations are 4.3 g/L for ethylene glycol and 5.3 g/L for glycolic acid 5
- Urine-to-blood ratios are approximately 3.8 for ethylene glycol and 3.1 for glycolic acid, making urine an excellent screening matrix 5
- Urine should be the primary matrix for screening due to these high concentration ratios 5
Clinical Context and Timing
Early vs. Late Findings
- Oxalate crystals typically appear 3-6 hours after ingestion, coinciding with the onset of metabolic acidosis 2
- Early presentation may show only polyuria and polydipsia before crystal formation occurs 6
- The absence of crystals does not exclude ethylene glycol poisoning, especially in early presentation 1
Associated Laboratory Abnormalities
- Anion gap metabolic acidosis (anion gap >27 mmol/L indicates severe poisoning requiring hemodialysis) 2
- Increased osmolar gap (>50 suggests hemodialysis with fomepizole; >10 mandates hemodialysis if no antidote available) 2
- Blood glycolate concentration >12 mmol/L warrants hemodialysis 2
Critical Clinical Pitfall
Do not wait for oxalate crystals to appear before initiating treatment. Treatment with fomepizole should begin immediately upon suspicion based on history, anion gap metabolic acidosis, increased osmolar gap, or visual disturbances, even before crystals are detected 1. The FDA label explicitly states to begin treatment "immediately upon suspicion" rather than waiting for confirmatory urine findings 1. Delayed treatment waiting for crystal confirmation can result in irreversible acute renal failure and permanent organ damage 6, 3.