About Foscarnet Dosing
This information supports the Foscarnet Dose Calculator, a tool designed for clinicians to determine appropriate adult dosing regimens based on renal function and indication. Foscarnet is an antiviral medication used for cytomegalovirus (CMV) retinitis in patients with AIDS and for acyclovir-resistant mucocutaneous herpes simplex virus (HSV) infections in immunocompromised patients. Dosing must be individualized and requires careful calculation of creatinine clearance (CrCl).
Calculator Outputs Explained
After processing the inputs, the calculator provides a comprehensive dosing recommendation that includes:
- Dosing Weight: The patient weight (in kg) used for the CrCl calculation. This may be the actual, ideal, or adjusted body weight, depending on the patient's parameters, to ensure accurate dosing.
- Creatinine Clearance (CrCl): The estimated renal function in mL/min, calculated using the Cockcroft-Gault equation. This value is critical for determining the correct Foscarnet dose.
- Recommended Dose: The specific dose in mg/kg, the total dose in mg per administration, and the dosing frequency (e.g., every 8, 12, or 24 hours).
- Infusion Duration: The minimum recommended time for the infusion, typically 1 to 2 hours, to minimize the risk of nephrotoxicity.
How to Use the Calculator
To ensure an accurate dose calculation, please provide the following patient data:
- Indication: Select the reason for treatment (CMV Induction, CMV Maintenance, or Acyclovir-Resistant HSV).
- Age: Enter the patient's age in years.
- Weight & Height: Input the patient's actual weight and height. Use the toggles to switch between metric (kg, cm) and imperial (lbs, in) units.
- Serum Creatinine (SCr): Enter the most recent stable serum creatinine value. The unit can be toggled between mg/dL and µmol/L.
- Biological Sex: Select the patient's biological sex, as this is a variable in the Cockcroft-Gault formula.
Dosing Overview
Foscarnet dosing is stratified by indication and adjusted based on calculated creatinine clearance. The primary goal is to balance antiviral efficacy with the risk of renal toxicity.
- CMV Retinitis - Induction: Typically involves a higher dose (e.g., 60 mg/kg) administered more frequently (e.g., every 8 hours) for 2-3 weeks.
- CMV Retinitis - Maintenance: Following induction, the dose is reduced (e.g., 90-120 mg/kg) and administered less often (e.g., once daily) for long-term suppression.
- Acyclovir-Resistant HSV: Usually requires a dose of 40 mg/kg administered every 8 to 12 hours for 2-3 weeks or until lesions heal.
- Renal Impairment: As renal function declines (lower CrCl), both the dose (mg/kg) and the dosing interval are adjusted to prevent drug accumulation and toxicity. The calculator automates these adjustments based on established guidelines.
Switching Therapy
When switching to or from foscarnet, clinical judgment is paramount. There is no standard washout period required, but the patient's renal function, electrolyte balance, and hydration status must be stable before initiating foscarnet. When switching from another nephrotoxic agent, consider allowing renal function to return to baseline if possible. Always consult institutional protocols and infectious disease specialists for guidance on complex treatment changes.
Missed Dose Protocol
If a dose of foscarnet is missed, the patient should contact their healthcare provider for instructions. The decision on how to proceed depends on the indication (induction vs. maintenance), the time elapsed since the missed dose, and the patient's clinical status. Patients should be advised not to administer a double dose to make up for a missed one, as this can increase the risk of acute kidney injury and electrolyte abnormalities.
Safety Alerts
NEPHROTOXICITY: Foscarnet is a major cause of renal impairment. Regular monitoring of serum creatinine is mandatory (2-3 times per week during induction, at least weekly during maintenance). Adequate hydration, including pre-infusion saline loading, is crucial to mitigate this risk.
ELECTROLYTE ABNORMALITIES: The drug can cause severe, sometimes fatal, disturbances in calcium, phosphate, magnesium, and potassium levels. Frequent monitoring of electrolytes is essential, especially during induction therapy.
Frequently Asked Questions
The Cockcroft-Gault (CG) equation was used in the original clinical trials for Foscarnet to establish dosing adjustments for renal impairment. Therefore, the FDA-approved prescribing information bases its recommendations on CG-derived CrCl. Using other equations like MDRD or CKD-EPI may yield different results and is not recommended for Foscarnet dosing unless specified by institutional policy.
Dosing weight is the weight used in the CrCl formula to prevent over- or under-dosing in patients at extremes of weight. The calculator uses standard logic: if the patient is underweight (actual < ideal body weight), actual weight is used. If they are obese (actual > 120% of ideal), an adjusted body weight is used. Otherwise, actual body weight is used.
As indicated by the calculator, Foscarnet use is generally not recommended when the calculated creatinine clearance per kilogram of body weight falls below 0.4 mL/min/kg. In such cases, the risks of severe toxicity are very high. Alternative therapies should be strongly considered, and an infectious disease or nephrology specialist should be consulted.
No. This calculator is designed for adult patients only. The safety, efficacy, and pharmacokinetics of Foscarnet have not been established in pediatric patients, and dosing would require specialized pediatric infectious disease consultation.
Foscarnet can precipitate in the renal tubules, leading to acute kidney injury. Co-administration of 750-1000 mL of 0.9% NaCl (normal saline) or 5% Dextrose in Water (D5W) before the foscarnet infusion increases urine flow and dilutes the drug in the tubules, significantly reducing the risk of nephrotoxicity.
The Foscarnet dose should be re-calculated whenever there is a significant change in the patient's renal function (serum creatinine). The prescribing information recommends monitoring SCr 2-3 times per week during induction and at least once a week during maintenance. Any change in SCr necessitates a dose re-assessment.
No. This calculator is not designed for patients on any form of renal replacement therapy, including hemodialysis or peritoneal dialysis. Dosing in this population is complex and requires specialist consultation, as Foscarnet is significantly cleared by dialysis.
Induction therapy is a high-dose, short-term (2-3 week) regimen designed to quickly gain control of a viral infection like CMV retinitis. Once the infection is controlled, the patient transitions to a lower-dose, less frequent maintenance therapy to prevent the virus from reactivating.
References
- FOSCAVIR (foscarnet sodium) Injection Prescribing Information. U.S. Food and Drug Administration. View PDF
- DailyMed - FOSCAVIR- foscarnet sodium injection, solution. National Library of Medicine. Visit Page
- Guidelines for the Prevention and Treatment of Opportunistic Infections in Adults and Adolescents with HIV. Centers for Disease Control and Prevention (CDC). Visit Page
- Cockcroft DW, Gault MH. Prediction of creatinine clearance from serum creatinine. Nephron. 1976;16(1):31-41. View on PubMed
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