Ethylene Glycol Toxicosis in Dogs and Cats: Clinical Management

Bottom line

Ethylene glycol (EG) itself is relatively benign; the toxicity is metabolic. Hepatic alcohol dehydrogenase (ADH) converts EG to glycolaldehyde → glycolic acid → glyoxylic acid → oxalic acid, driving a severe high-anion-gap metabolic acidosis, while oxalate binds calcium and precipitates as calcium oxalate crystals in the renal tubules, producing acute kidney injury (AKI). EG is absorbed rapidly, peaking at 1–4 hours. [1][2]

Management is an antidote race: inhibit ADH before significant metabolism. Fomepizole (4-methylpyrazole) is the antidote of choice; ethanol is the alternative. The window is narrow — roughly within 8 hours in dogs and within 3 hours in cats. Prognosis is excellent when the antidote is started pre-azotemia and grave once the patient is oliguric or anuric. [1][3][4]

Clinical facts

• Minimum lethal dose (undiluted EG): dogs 4.4–6.6 mL/kg; cats 1.4 mL/kg. Cats are dramatically more sensitive. [1][2]
• Antidote window: dogs — fomepizole is most effective within ~5–8 hours of ingestion (2/6 dogs treated at 8 h still developed AKI); cats — within 3 hours (survival if treated ≤3 h; AKI in cats treated ≥4 h regardless of antidote). Once azotemic, antidotal therapy alone is often too late. [3][4]
• Peak absorption: 1–4 hours after ingestion; plasma half-life ~3–5 h (prolonged to 12–72 h by ADH inhibition). [1][2]
• Key labs at a glance: high osmolar gap (>20 mOsm/kg) and high anion gap metabolic acidosis (>25 mEq/L) within ~3 h; calcium oxalate monohydrate crystalluria (as early as 3 h in cats, 6 h in dogs); hypocalcemia; isosthenuria with acidic pH. [1][2]

Toxicokinetics and mechanism

EG is a component of automotive antifreeze, radiator coolant, and some de-icers, and is palatable to dogs and cats. It is absorbed rapidly from the GI tract with peak blood concentrations at 1–4 hours after ingestion. [1][2]

Parent EG is only mildly toxic — an osmotically active alcohol that causes early CNS "inebriation" and osmotic diuresis. The lethality comes from hepatic biotransformation: alcohol dehydrogenase oxidizes EG to glycolaldehyde, which is converted to glycolic acid, then glyoxylic acid, and finally oxalic acid. Glycolic acid is the principal driver of the profound high-anion-gap metabolic acidosis, and oxalic acid chelates calcium to precipitate as calcium oxalate crystals within the proximal renal tubular epithelium, causing tubular necrosis and AKI. [1][2] Because the whole cascade depends on ADH, blocking that first enzymatic step is the entire rationale for antidotal therapy.

The minimum lethal dose of undiluted EG (typically 95–97% by volume in antifreeze) is 4.4–6.6 mL/kg in dogs and just 1.4 mL/kg in cats — the roughly 3- to 4-fold greater feline sensitivity is central to every triage and prognosis decision below. [1][2]

Clinical stages

Signs progress through overlapping stages; the timeline is compressed in cats. [1][2]

• Stage 1 — neurologic / "inebriation" + osmotic diuresis (≈30 min to 12 h): nausea, vomiting, ataxia, CNS depression, weakness, and marked polyuria/polydipsia from osmotic diuresis. This is the window in which antidotal therapy is most effective, yet signs are easily mistaken for other causes of acute encephalopathy or simple GI upset. [1][2]
• Stage 2 — cardiopulmonary (≈12 to 24 h): worsening severe metabolic acidosis, tachypnea/hyperpnea, tachycardia, and sometimes seizures as glycolic-acid accumulation peaks. [2]
• Stage 3 — oliguric/anuric AKI: in dogs generally 24–72 h after ingestion; in cats as early as 12–24 h. Azotemia, oliguria progressing to anuria, and often severe uremia. Reaching this stage markedly worsens prognosis. [1][2]

Diagnosis and clinicopathology

Time is the enemy of a clean diagnosis — the most treatable window is also when confirmatory testing is least definitive.

• Osmolar gap: an early, sensitive clue. Marked serum hyperosmolality with an increased osmolar gap (>20 mOsm/kg) appears within ~3 h of ingestion, before metabolites accumulate. [1][2]
• High-anion-gap metabolic acidosis: an increased anion gap (>25 mEq/L) with decreased blood pH develops within ~3 h as glycolic acid accumulates — one of the most consistent early findings. [1][2]
• In-house EG blood test: positive results are meaningful, but the assay has a narrow positive window (approximately 0.5–12 h after ingestion) and negative results do not rule out toxicosis — false negatives are common late (metabolism has cleared parent EG) and in cats. Do not withhold antidote on a negative test if the history and biochemistry fit. [2]
• Calcium oxalate monohydrate crystalluria: supportive when present — seen as early as 3 h in cats and 6 h in dogs — but absence does not exclude EG. [1][2]
• Hypocalcemia: consistent with oxalate–calcium chelation and supports the diagnosis. [1][2]
• Wood's-lamp fluorescence: some antifreeze products contain fluorescein, so urine, vomitus, or perioral fur may fluoresce (typically only up to ~6 h). It is unreliable — not all products are dyed, background fluorescence is common, and a negative result never excludes exposure. [2]
• Renal ultrasound "halo" sign: a late finding — diffusely increased renal cortical echogenicity, often with a hyperechoic band producing a halo around the medulla — seen once tubular calcium oxalate deposition is established, by which point the prognosis has already worsened. [5]

Antidotal therapy: fomepizole and ethanol

The therapeutic goal is to competitively inhibit ADH and halt metabolism until parent EG is renally excreted. Start the antidote as early as possible — before significant metabolism (roughly within 8 h in dogs, 3 h in cats) — and do not wait for a confirmatory test. [1][3][4]

Fomepizole (4-methylpyrazole) — preferred. It inactivates ADH without the CNS depression, hyperosmolality, and acidosis that ethanol adds. [1][2]

• Dogs (Merck formulary regimen, 5% [50 mg/mL] solution): 20 mg/kg IV initially, then 15 mg/kg IV at 12 and 24 hours, then 5 mg/kg IV at 36 hours. [1] (The original canine efficacy study used a 20 / 15 / 10 / 5 mg/kg taper at 0 / 12 / 24 / 30 h and showed dogs treated at 5 h recovered without renal injury, while 2/6 treated at 8 h developed AKI. [3])
• Cats (extra-label high-dose protocol): cats require a substantially higher dose than dogs — 125 mg/kg IV initially, then 31.25 mg/kg IV at 12, 24, and 36 hours. In the controlled feline study, high-dose fomepizole was safe, prevented fatal AKI when instituted within 3 hours of ingestion, and was more effective than ethanol. [1][4]

Ethanol CRI — alternative when fomepizole is unavailable. Ethanol competes with EG for ADH but worsens CNS depression and the metabolic acidosis, adds to hyperosmolality, and is harder to titrate. One cited canine regimen: 30% ethanol at 1.3 mL/kg IV bolus, then 0.42 mL/kg/h CRI for 48 hours. [1] A feline protocol from the comparative study: 5 mL/kg of 20% ethanol IV initially, then every 6 hours for 5 treatments, then every 8 hours for 4 treatments; in that study ethanol was less effective than high-dose fomepizole in cats. [4]

In all cases, the antidote is a race against ADH: once the patient is azotemic, EG has largely been metabolized and ADH inhibition alone cannot reverse established oxalate nephrosis — the focus shifts to supportive care and extracorporeal removal. [3][4]

Supportive care and hemodialysis

Antidotal therapy runs alongside aggressive supportive care. [1][2]

• IV fluid diuresis: an isotonic alkalinizing crystalloid (e.g., lactated Ringer's) at roughly twice maintenance to promote EG excretion and support renal perfusion, titrated to volume status and urine output. [2]
• Correct metabolic acidosis: sodium bicarbonate for severe acidemia (e.g., blood pH <7.10–7.15 or total CO₂ <10–12 mEq/L), guided by serial blood gases. [2]
• Monitor and support calcium: watch for symptomatic hypocalcemia from oxalate chelation. [1][2]
• Hemodialysis / extracorporeal therapy: in established renal failure, hemodialysis is the key intervention — it clears both parent EG and its toxic metabolites and manages uremia, hyperkalemia, and fluid overload during the oligoanuric phase. It is considered superior to peritoneal dialysis for eliminating EG and metabolites, though availability in veterinary practice remains limited; early referral to a facility with renal-replacement capability is warranted for azotemic or anuric patients. [2]

Monitoring and prognosis

Serially track acid–base status, anion and osmolar gaps, renal values, electrolytes (calcium, potassium), and — critically — urine output to detect the transition to oliguria/anuria. [1][2]

Prognosis hinges almost entirely on how early the antidote is started relative to metabolism. It is excellent when treatment begins pre-azotemia and within the species window (≤~8 h in dogs, ≤3 h in cats). Once the patient is azotemic, and especially once oliguric or anuric, the prognosis is guarded to grave; crystalluria, oliguric azotemia, and anuria are poor prognostic indicators, and reported mortality is high — roughly 44–70% in dogs and up to ~96–97% in cats. Hemodialysis can salvage some established-AKI patients but does not restore the excellent outlook of the pre-azotemic window. [1][2]

Frequently Asked Questions

What is the fomepizole protocol for ethylene glycol toxicity in dogs?

The standard canine regimen (Merck Veterinary Manual) uses fomepizole (4-methylpyrazole) as a 5% [50 mg/mL] solution: 20 mg/kg IV initially, then 15 mg/kg IV at 12 and 24 hours, then 5 mg/kg IV at 36 hours. Fomepizole is preferred over ethanol because it inhibits alcohol dehydrogenase without adding CNS depression or acidosis. [1]

Why do cats need a higher fomepizole dose than dogs?

Cats are far more sensitive to ethylene glycol and, at standard canine doses, fomepizole was historically considered ineffective in cats. A controlled feline study established a high-dose protocol — 125 mg/kg IV initially, then 31.25 mg/kg IV at 12, 24, and 36 hours — that is safe and prevents fatal acute renal failure when started within 3 hours of ingestion, and that outperformed ethanol. The higher dose is required to achieve sufficient ADH inhibition in cats; note it is an extra-label regimen. [1][4]

What is the minimum lethal dose of ethylene glycol in cats versus dogs?

The minimum lethal dose of undiluted ethylene glycol is approximately 1.4 mL/kg in cats and 4.4–6.6 mL/kg in dogs. Cats' roughly 3- to 4-fold greater sensitivity means even a tiny volume — a few licks of spilled antifreeze or a paw groomed after walking through it — can be lethal. [1][2]

How long is the treatment window for ethylene glycol toxicity?

Because the antidote works by blocking metabolism, it must be given before significant ethylene glycol is metabolized — roughly within 8 hours in dogs and within 3 hours in cats. In dogs, treatment at 5 hours prevented renal injury while 2 of 6 treated at 8 hours still developed AKI; in cats, survival required treatment within 3 hours, and cats treated at 4 hours or later developed acute renal failure regardless of antidote. Once the patient is azotemic, the parent compound has largely been metabolized and antidotal therapy alone is often too late. [3][4]

When is ethanol used instead of fomepizole?

Ethanol is the alternative antidote when fomepizole is unavailable. It also competes for alcohol dehydrogenase but has real downsides: it worsens CNS depression and metabolic acidosis, adds to hyperosmolality, and is harder to titrate, requiring closer monitoring. One canine regimen is 30% ethanol at 1.3 mL/kg IV bolus, then 0.42 mL/kg/h CRI for 48 hours; a feline protocol is 5 mL/kg of 20% ethanol IV initially, then every 6 hours for 5 doses, then every 8 hours for 4 doses. In the comparative feline study, ethanol was less effective than high-dose fomepizole. [1][4]

Does a negative in-house ethylene glycol test rule out poisoning?

No. The in-house EG assay has a narrow positive window (roughly 0.5–12 hours after ingestion), and negative results do not exclude toxicosis — false negatives are common late (parent EG has been metabolized) and in cats. If the history, high osmolar gap, and high-anion-gap metabolic acidosis fit, start the antidote without waiting for confirmation. [2]

What is the halo sign on renal ultrasound in ethylene glycol toxicity?

The "halo" sign is diffusely increased renal cortical echogenicity, often with a hyperechoic band producing a halo appearance around the medulla, caused by calcium oxalate deposition in the renal tubules. It is a late finding — by the time it appears, tubular injury is established and the prognosis has already worsened — so it confirms rather than enables early intervention. [5]

What is the prognosis for ethylene glycol toxicosis?

Prognosis is excellent when the antidote is started pre-azotemia and within the species window (≤~8 h dogs, ≤3 h cats). It becomes guarded to grave once the patient is azotemic, and especially once oliguric or anuric — crystalluria, oliguric azotemia, and anuria are poor prognostic indicators. Reported mortality is roughly 44–70% in dogs and up to ~96–97% in cats. Hemodialysis can rescue some established-AKI patients but does not match the outcome of the pre-azotemic window. [1][2]