About This Tool

The Fomepizole Dosing Calculator is a clinical support tool designed to simplify dose calculations for patients with confirmed or suspected methanol or ethylene glycol poisoning. It provides patient-specific dosing for both the initial loading dose and subsequent maintenance infusions, including critical adjustments for patients undergoing hemodialysis.

Outputs

Upon entering the patient’s information, the calculator provides the following outputs:

• Loading Dose: The initial, weight-based dose in both milligrams (mg) and volume (mL) to quickly achieve therapeutic concentrations.
• Maintenance Doses: A clear, scheduled regimen for ongoing treatment, accounting for fomepizole’s auto-induction of metabolism.
• Hemodialysis Adjustments: A modified dosing schedule with increased frequency to compensate for the drug’s removal during a hemodialysis session.
• Administration Notes: Important reminders regarding dilution and infusion time for safe administration.

How to Use

1. Enter Patient Weight: Input the patient’s body weight. The tool accepts both kilograms (kg) and pounds (lbs).
2. Select Renal/HD Status: Choose the option that reflects the patient’s current situation: normal renal function (no HD) or currently undergoing hemodialysis.
3. Calculate: The tool instantly displays the complete, recommended dosing regimen based on the provided inputs.

Dosing Overview

Fomepizole dosing follows a specific protocol to effectively inhibit alcohol dehydrogenase:

• Loading Dose: A single dose of 15 mg/kg is administered intravenously.
• Standard Maintenance: Begins 12 hours after the loading dose. The first four maintenance doses are 10 mg/kg every 12 hours.
• Increased Maintenance: Due to auto-induction, subsequent doses are increased to 15 mg/kg every 12 hours to maintain therapeutic levels.
• Hemodialysis: Fomepizole is significantly cleared by dialysis. The dosing interval is shortened to every 4 hours for the duration of the dialysis session to prevent sub-therapeutic levels.

Treatment Considerations

Fomepizole is the preferred antidote over ethanol due to its more predictable pharmacokinetics, lack of CNS depression, and easier administration. Treatment should continue until toxic alcohol concentrations are below toxic levels (e.g., <20 mg/dL) and the patient is free of metabolic acidosis. Hemodialysis is a critical adjunct therapy to remove the parent alcohol and its toxic acidic metabolites.

Missed Dose

In the controlled setting where fomepizole is used, missed doses are uncommon but should be addressed promptly. If a maintenance dose is delayed, it should be administered as soon as remembered, and the 12-hour interval should be reset from that time. For patients on hemodialysis, the timing of doses relative to the end of a session is critical; refer to the calculator’s post-HD guidance for specific instructions.

Safety Alerts

• Administration: Fomepizole must be diluted (e.g., in 100 mL of NS or D5W) and infused slowly over 30 minutes to minimize the risk of venous irritation and phlebitis.
• Adverse Effects: Common side effects include headache, nausea, and dizziness. Serious adverse events are rare.
• Monitoring: Regular monitoring of acid-base status, renal function, and toxic alcohol levels is essential throughout treatment.

Frequently Asked Questions

1. Why does the maintenance dose increase after 48 hours?
   Fomepizole induces its own metabolism over time. The dose is increased from 10 mg/kg to 15 mg/kg after the first four maintenance doses to counteract this effect and maintain effective drug concentrations.
2. What concentration does the calculator use to determine the volume in mL?
   The calculator assumes a standard final concentration of 25 mg/mL after dilution, as specified in the administration notes.
3. Why is dosing more frequent during hemodialysis?
   Fomepizole is a small molecule that is effectively removed by hemodialysis. Dosing every 4 hours is necessary to replace the drug lost during the procedure and maintain its therapeutic effect.
4. Can this calculator be used for pediatric patients?
   This calculator is designed for adult dosing. Pediatric dosing may differ, and consultation with a pediatric toxicologist or poison control center is required.
5. What is the purpose of the loading dose?
   The loading dose is larger than the maintenance doses to rapidly achieve a therapeutic blood concentration of fomepizole, which is necessary to quickly and completely inhibit alcohol dehydrogenase.
6. Is it acceptable to round the calculated dose?
   Rounding the dose to the nearest convenient vial size or milligram amount is a common practice based on institutional policy. Always consult a pharmacist for guidance on dose rounding.
7. What if the patient’s exact weight is unknown?
   Use an estimated weight to begin therapy promptly. The dose can be adjusted if a more accurate weight becomes available. In toxic alcohol poisoning, prompt treatment is critical.
8. Does this tool replace the need to consult a poison control center?
   No. This tool is for educational and support purposes only. Management of a poisoned patient is complex and should always be done in consultation with a regional poison control center or a medical toxicologist.

References

The dosing principles are based on established clinical guidelines and prescribing information.

1. Antizol (fomepizole) Injection Prescribing Information. U.S. Food and Drug Administration.
2. Barceloux DG, Krenzelok EP, Olson K, Watson W. American Academy of Clinical Toxicology practice guidelines on the treatment of ethylene glycol poisoning. J Toxicol Clin Toxicol. 1999;37(5):537-60.
3. Brent J. Fomepizole for ethylene glycol and methanol poisoning. N Engl J Med. 2009;360(21):2216-23. doi:10.1056/NEJMct0806112
4. Lepik KJ, Levy AR, Sobolev BG, et al. Adverse drug events associated with the antidotes for methanol and ethylene glycol poisoning: a comparison of ethanol and fomepizole. Ann Emerg Med. 2009;53(4):439-450.e9.