Voriconazole Dosing for a 15-Year-Old Adolescent Weighing 55 kg
For a 15-year-old adolescent weighing 55 kg, use the standard adult voriconazole dosing regimen regardless of body weight: 200 mg orally twice daily for maintenance therapy (or 4 mg/kg IV twice daily), with a loading dose of 400 mg orally twice daily for 2 doses (or 6 mg/kg IV twice daily for 2 doses) on day 1. 1, 2
Age-Based Dosing Transition at 15 Years
At exactly 15 years of age, adult dosing applies regardless of body weight, distinguishing this patient from younger adolescents where weight thresholds matter. 1, 2
For patients aged 12-14 years, adult dosing only applies if they weigh ≥50 kg; however, once a patient reaches 15 years, the weight criterion becomes irrelevant and adult dosing is mandatory. 1, 2
This represents a critical transition point: pediatric weight-based dosing (8-9 mg/kg twice daily used in children 2-14 years) is replaced by fixed adult dosing at age 15. 1
Specific Dosing Regimen by Route and Indication
Oral Administration (Preferred After Clinical Stability)
Loading dose: 400 mg (6 mg/kg) orally twice daily for 2 doses on day 1. 2
If inadequate response occurs, the maintenance dose may be increased to 300 mg orally twice daily. 2
Oral bioavailability exceeds 90%, making oral and IV formulations interchangeable once the patient is clinically stable. 1, 2
Intravenous Administration (For Severe Infections or Inability to Take Oral)
Loading dose: 6 mg/kg IV twice daily for 2 doses on day 1 (first 24 hours). 1, 2
If inadequate response occurs, continue at 4 mg/kg IV twice daily; if intolerance develops, reduce to 3 mg/kg IV twice daily. 2
Critical caveat: Avoid IV voriconazole if creatinine clearance <50 mL/min due to cyclodextrin accumulation risk, though recent data suggest this may be overly cautious. 1, 2
Indication-Specific Considerations
For invasive aspergillosis: Use the full loading and maintenance regimen as above, with median IV therapy duration of 10 days followed by oral therapy (median 76 days in clinical trials). 1, 2
For candidemia and invasive candidiasis: The same loading dose applies, but maintenance can be either 3-4 mg/kg IV or 200 mg orally twice daily depending on severity. 1, 2
For prophylaxis in high-risk settings (AML, allogeneic HSCT): Use 200 mg orally twice daily without loading dose. 1
Therapeutic Drug Monitoring is Essential
Target trough concentration: ≥1 mg/L for efficacy, with toxicity risk increasing above 4-5 mg/L. 1, 3, 4, 5
Draw trough levels before the morning dose at steady state (typically after 5-7 days), though earlier monitoring may be warranted in critically ill patients. 4, 5, 6
Young adolescents (12-14 years) with low body weight may metabolize voriconazole more like children than adults, potentially requiring higher doses to achieve therapeutic levels, but this 15-year-old at 55 kg should follow standard adult dosing. 7
Approximately 62% of patients achieve therapeutic levels with initial dosing; dose adjustments based on levels can increase this to 80% by the second adjustment. 4
Critical Drug Interactions and Monitoring
CYP2C19 polymorphisms significantly affect voriconazole metabolism: Poor metabolizers have 4-fold higher exposure, while ultra-rapid metabolizers may have subtherapeutic levels. 5, 6
Avoid coadministration with rifampin, carbamazepine, long-acting barbiturates, efavirenz (standard dose), ritonavir, and St. John's Wort due to loss of efficacy. 2
Avoid coadministration with cisapride, pimozide, quinidine, and sirolimus due to risk of serious adverse reactions including QT prolongation. 2
Proton pump inhibitors and H2-receptor antagonists do not significantly affect voriconazole absorption (unlike itraconazole), but pantoprazole use may alter pharmacokinetics. 6
Common Pitfalls to Avoid
Do not use pediatric weight-based dosing (8-9 mg/kg twice daily) for this 15-year-old, even though the calculated dose would be similar; the guideline explicitly mandates adult fixed dosing at this age. 1, 2
Do not skip the loading dose for treatment of active infections, as time to steady state can be 5-7 days without it, which is suboptimal for critically ill patients. 2, 6
Do not assume therapeutic levels are achieved without monitoring; younger adolescents and those with low BMI (≤25 kg/m²) or age ≤30 years have higher rates of subtherapeutic levels. 7, 4
Monitor for visual disturbances (occur in ~30% of patients), hepatotoxicity, photosensitivity, and QT prolongation; these are the most common adverse effects. 1, 8
Hepatic and Renal Dose Adjustments
Mild to moderate hepatic impairment (Child-Pugh A or B): Use standard loading dose but reduce maintenance dose by 50% (to 100 mg orally twice daily or 2 mg/kg IV twice daily). 2
Severe hepatic impairment (Child-Pugh C): No established dosing; use only if benefit outweighs risk with intensive monitoring. 2
Renal impairment: No oral dose adjustment needed; avoid IV formulation if creatinine clearance <50 mL/min. 2