What are the recommended loading and maintenance doses of fluconazole and voriconazole for invasive candidiasis and invasive aspergillosis, and how should these doses be adjusted in patients with impaired renal function (creatinine clearance ≤50 mL/min) or hepatic impairment (Child‑Pugh A‑C)?

Fluconazole and Voriconazole Dosing for Invasive Fungal Infections

Fluconazole Dosing

Standard Dosing for Invasive Candidiasis

For invasive candidiasis and candidemia, administer a loading dose of 800 mg (12 mg/kg) on day 1, followed by 400 mg (6 mg/kg) daily for at least 14 days after the first negative blood culture and resolution of symptoms. 1, 2

• The loading dose is critical to rapidly achieve therapeutic concentrations given fluconazole's long half-life of approximately 30 hours 2
• Treatment duration must extend at least 2 weeks beyond documented clearance of Candida from bloodstream 2, 3

Site-Specific Dosing Variations

CNS Candidiasis:

• Use 400-800 mg (6-12 mg/kg) daily after initial amphotericin B therapy 1
• Higher doses are needed due to the severity of infection despite good CSF penetration 1

Esophageal Candidiasis:

• 200-400 mg (3-6 mg/kg) daily for 14-21 days 1, 2
• Use 400 mg daily for moderate-to-severe disease 2

Urinary Tract Candidiasis:

• Symptomatic cystitis: 200 mg (3 mg/kg) daily for 14 days 1, 2
• Pyelonephritis: 200-400 mg (3-6 mg/kg) daily for 14 days 1, 2
• Fluconazole achieves urinary concentrations 10-20 times higher than serum levels, making it ideal for urinary infections 2

Renal Dose Adjustments

The critical threshold for dose reduction is creatinine clearance ≤50 mL/min (not 60 mL/min). 2

• Always give the full loading dose (800 mg) regardless of renal function 2
• Reduce maintenance dose by 50% when CrCl ≤50 mL/min (e.g., 200 mg daily instead of 400 mg) 2
• Hemodialysis patients: Administer 400 mg after each dialysis session (typically 3 times weekly), as approximately 50% of fluconazole is removed during a 3-hour session 2

Hepatic Impairment

• No dose adjustment is required for hepatic impairment 2
• Fluconazole is primarily renally eliminated (>90% unchanged in urine), making hepatic dysfunction irrelevant to dosing 2, 4

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Voriconazole Dosing

Standard Dosing for Invasive Aspergillosis

For invasive aspergillosis, administer a loading dose of 400 mg (6 mg/kg) every 12 hours for two doses on day 1, followed by a maintenance dose of 200-300 mg (3-4 mg/kg) twice daily. 1, 5

• The IV loading dose is 6 mg/kg every 12 hours for the first 24 hours (420 mg IV every 12 hours for a 70 kg patient) 5
• The IV maintenance dose is 3-4 mg/kg every 12 hours (210-280 mg IV every 12 hours for a 70 kg patient) 5
• Oral bioavailability exceeds 90%, allowing seamless transition between IV and oral formulations 5

Route Selection Based on Renal Function

Avoid IV voriconazole in patients with creatinine clearance <50 mL/min due to accumulation of the nephrotoxic vehicle sulfobutylether-β-cyclodextrin (SBECD). 5, 6

• Use oral voriconazole instead at standard doses without adjustment in renal impairment 5, 6
• The oral formulation does not contain SBECD and requires no renal dose adjustment 5

Hepatic Dose Adjustments

Voriconazole is the only triazole requiring dose reduction in hepatic impairment. 5, 6

• Reduce the maintenance dose by 50% in patients with mild to moderate hepatic impairment (Child-Pugh Class A or B) 5, 6
• Keep the loading dose unchanged 5
• Example: For a patient with Child-Pugh B cirrhosis, give 400 mg every 12 hours × 2 doses, then 100 mg (instead of 200 mg) every 12 hours

Administration Considerations

• Take oral voriconazole at least 1 hour before or after meals to optimize absorption, as bioavailability decreases with food 5
• Maximum IV infusion rate is ≤200 mg per hour to avoid infusion reactions 2
• Therapeutic drug monitoring is beneficial due to high interpatient variability from CYP2C19 polymorphisms 5

Pediatric Dosing

• Higher doses are required in children due to accelerated metabolic clearance 5
• The European Medicines Agency recommends 7 mg/kg twice daily maintenance dosing in pediatric patients to achieve plasma levels comparable to adults 5

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Critical Species-Specific Considerations

Fluconazole Resistance Patterns

• Candida krusei is intrinsically resistant to fluconazole—use amphotericin B or an echinocandin instead 2, 3
• Candida glabrata may require higher fluconazole doses or alternative agents due to reduced susceptibility 2

When to Favor Fluconazole vs. Echinocandins

• Favor fluconazole for patients who are not critically ill and unlikely to have fluconazole-resistant species 1
• Favor an echinocandin for moderate-to-severe illness, recent azole exposure, or high risk of C. glabrata or C. krusei infection 1

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Common Pitfalls and How to Avoid Them

Fluconazole Pitfalls

• Do not reduce the loading dose in renal failure—only the maintenance dose requires adjustment 2
• Do not use fluconazole for urinary candidiasis caused by C. krusei—it is intrinsically resistant 2, 3
• Do not forget to remove central venous catheters in candidemia, as antifungals alone are insufficient 2, 3

Voriconazole Pitfalls

• Do not use IV voriconazole in patients with CrCl <50 mL/min—switch to oral formulation 5, 6
• Do not forget to halve the maintenance dose in hepatic impairment—voriconazole is unique among triazoles in requiring this adjustment 5, 6
• Monitor for visual disturbances (occur in ~30% of patients), hepatotoxicity, photosensitivity, and QTc prolongation 5
• Evaluate drug interactions carefully—voriconazole inhibits CYP3A4, CYP2C19, and CYP2C9, requiring dose reductions of warfarin (by 50%), cyclosporine, and tacrolimus 5