Cats are approximately 4 times more sensitive to ethylene glycol than dogs, with a minimum lethal dose of just 1.4 mL/kg compared to 4.4 mL/kg in dogs. For a 4 kg cat, ingesting less than 6 mL of antifreeze (about one teaspoon) can be fatal. The sweet taste makes it palatable to both species. Use the Serum Osmolarity Calculator for early osmolal gap assessment and the Toxicology Specialist for treatment guidance.
Ethylene glycol (EG) itself has relatively low toxicity, producing primarily CNS depression similar to ethanol intoxication. The danger lies in its metabolites. Alcohol dehydrogenase (ADH) converts EG to glycolaldehyde, which is further metabolized to glycolic acid (the primary cause of severe metabolic acidosis), glyoxylic acid, and finally oxalic acid (which chelates calcium to form calcium oxalate crystals that deposit in renal tubules).
The treatment rationale for fomepizole and ethanol centers on competitively inhibiting ADH, preventing the formation of these toxic metabolites. Unmetabolized EG is excreted renally with a half-life of approximately 3 hours. This is why early treatment (before significant metabolism has occurred) is dramatically more effective.
Ethylene glycol toxicity progresses through three distinct clinical stages, each corresponding to the accumulation of different metabolites and organ damage:
| Stage | Timeframe | Clinical Signs | Pathophysiology |
|---|---|---|---|
| Stage 1 | 0.5-12 hours | Ataxia, "drunken" gait, vomiting, PU/PD, CNS depression, nystagmus | CNS effects of parent compound (EG); osmotic diuresis from EG excretion |
| Stage 2 | 12-24 hours | Tachycardia, tachypnea, apparent clinical improvement, then progressive decline | Cardiopulmonary effects of metabolites; hypocalcemia from calcium oxalate formation |
| Stage 3 | 24-72 hours | Oliguria/anuria, severe depression, vomiting, seizures, coma, death | Oliguric acute renal failure from calcium oxalate crystal deposition in tubules |
Warning: Stage 2 often features an apparent clinical improvement that deceives owners into thinking the pet is recovering. This "lucid interval" occurs as CNS effects wane but before renal failure manifests. The patient is actively developing irreversible kidney damage during this period. Do not delay treatment based on apparent improvement.
Diagnosis combines clinical suspicion, laboratory findings, and specific testing. Key diagnostic clues include:
Elevated osmolal gap (early): Use the Serum Osmolarity Calculator to calculate. An osmolal gap >25 mOsm/kg in a symptomatic patient is highly suggestive. Peaks at approximately 6 hours post-ingestion. High anion gap metabolic acidosis (intermediate to late): AG often exceeds 40 mEq/L. Use the Blood Gas Interpreter for evaluation. Calcium oxalate crystalluria: Monohydrate crystals (six-sided prisms, "picket fence") appear as early as 3-6 hours; dihydrate crystals ("Maltese cross" or envelope-shaped) may also be seen. However, absence of crystals does not exclude EG toxicity.
Wood's lamp fluorescence: Some antifreeze products contain fluorescein dye, causing oral mucosa, vomitus, or urine to fluoresce under ultraviolet light. This test is unreliable (high false-negative and false-positive rates) and should never be used to rule out toxicity.
Ethylene glycol test kits: Point-of-care kits detect EG at >50 mg/dL. Sensitive in dogs (false positives possible with propylene glycol) but less reliable in cats due to the very small toxic dose. False negatives occur if tested too late (EG already metabolized).
Fomepizole (4-methylpyrazole, Antizol-Vet) is the preferred antidote. It competitively inhibits alcohol dehydrogenase with higher affinity than ethylene glycol, preventing formation of toxic metabolites. The treatment protocol differs between dogs and cats:
Dogs:
Loading dose: 20 mg/kg IV. At 12 hours: 15 mg/kg IV. At 24 hours: 15 mg/kg IV. At 36 hours: 5 mg/kg IV.
Cats:
Loading dose: 125 mg/kg IV. At 12 hours: 31.25 mg/kg IV. At 24 hours: 31.25 mg/kg IV. Then every 12 hours at 31.25 mg/kg until EG levels undetectable.
The dramatically higher fomepizole dose in cats (125 mg/kg vs 20 mg/kg in dogs) reflects the feline enzyme system differences. At canine doses, fomepizole is ineffective in cats. This higher dosing also makes feline treatment significantly more expensive, but it is far more effective than the ethanol alternative in cats.
When fomepizole is unavailable, ethanol can serve as an alternative antidote by competing for ADH. However, ethanol has significant drawbacks: profound CNS depression, respiratory depression, hyperosmolality, and hypoglycemia. It also makes clinical monitoring more difficult because its side effects mimic EG toxicity.
Ethanol protocol (dogs): 20% ethanol IV: 5.5 mL/kg bolus, then 1.1 mL/kg/hr CRI for 48 hours. Cats: 20% ethanol IV: 5 mL/kg bolus, then 0.8 mL/kg/hr CRI for 48 hours. Monitor blood glucose closely (ethanol causes hypoglycemia). Monitor respiratory rate and CNS depression. The patient will be profoundly sedated throughout treatment.
Warning: Ethanol therapy requires intensive monitoring and carries significant risks. Fomepizole is strongly preferred whenever available. Do not use vodka or other spirits orally as a substitute for calibrated IV ethanol solutions. Accurate concentration and dosing are critical.
Regardless of antidote choice, aggressive supportive care is essential: IV fluid therapy at 2-3 times maintenance to promote diuresis and renal excretion of EG and metabolites. Correct metabolic acidosis if pH <7.1-7.15 (bicarbonate therapy may be considered in severe acidosis refractory to fluid therapy, though it is controversial). Monitor and correct electrolytes: hypocalcemia (from calcium oxalate formation) may require calcium gluconate supplementation. Monitor urine output closely; oliguria (<1 mL/kg/hr) indicates renal failure.
Prognosis: Excellent if treated within 5-8 hours of ingestion (dogs) or 3 hours (cats) before significant metabolism occurs. Guarded to poor once oliguric renal failure develops. Hemodialysis can be pursued at referral centers but is limited in availability and costly. Once established anuric renal failure has occurred, prognosis is grave.
- Minimum lethal doses: dogs 4.4 mL/kg, cats 1.4 mL/kg; cats are approximately 4x more sensitive.
- Three stages: neurological (0-12h), cardiopulmonary (12-24h), oliguric renal failure (24-72h).
- Osmolal gap is elevated early (peaks ~6h); AG metabolic acidosis develops later as EG is metabolized.
- Fomepizole is the preferred antidote: 20 mg/kg loading dose in dogs, 125 mg/kg in cats.
- Treatment must begin before significant metabolism occurs; prognosis is poor once oliguric renal failure develops.
- Wood's lamp fluorescence and EG test kits are supplementary but unreliable as sole diagnostic tools.