What dosing regimen, adult or pediatric, should be applied for a 17-year-old female patient weighing 35 kg, considering potential renal dose adjustment?

Dosing Regimen for a 17-Year-Old Female Weighing 35 kg

Apply pediatric weight-based dosing for this 17-year-old patient weighing 35 kg, as she falls below the 40 kg threshold that typically defines the transition to adult dosing regimens. 1, 2

Key Dosing Principle

• The critical weight threshold is 40 kg, not age, for determining whether to use pediatric versus adult dosing regimens. 1, 2
• Children weighing more than 40 kg should be dosed as adults, but at 35 kg, this patient requires pediatric weight-based calculations (mg/kg). 1, 2
• Age alone (being 17 years old) does not automatically qualify a patient for adult dosing when body weight remains in the pediatric range. 1, 3

Specific Dosing Approach by Medication Class

For Most Medications

• Use weight-based dosing (mg/kg) calculated on actual body weight of 35 kg. 4, 3
• After 6 months of age through adolescence, body surface area (BSA) is appropriate for certain medications, but weight-based dosing remains standard for most drugs. 3
• At 35 kg, this patient's BSA would be approximately 1.13 m² (using the Mosteller formula), which should be used for chemotherapeutic agents or certain immunosuppressants if needed. 2

Antibiotic Examples

• Tuberculosis medications: Use pediatric weight-based dosing since she is under 40 kg—isoniazid 10-15 mg/kg daily (350-525 mg), rifampin 10-20 mg/kg daily (350-700 mg). 1, 4
• Intra-abdominal infections: Metronidazole 30-40 mg/kg/day divided every 8 hours (1,050-1,400 mg/day), ceftriaxone 50-75 mg/kg/day (1,750-2,625 mg/day). 4

Antiviral Example

• Oseltamivir for influenza: At 35 kg, use 60 mg twice daily (the >23-40 kg pediatric dosing bracket), not the adult 75 mg dose. 1, 2

Renal Dose Adjustment Considerations

• Calculate creatinine clearance to determine if renal dose adjustment is needed, using pediatric formulas appropriate for her age and weight. 1
• For drugs primarily renally excreted, adjust doses based on calculated renal function rather than assuming normal adult clearance. 3
• Lean body weight should be used for aminoglycoside dosing calculations if these agents are prescribed. 1

Critical Pitfalls to Avoid

• Do not assume adult dosing simply because the patient is 17 years old—weight is the determining factor. 1, 2
• Do not use ideal body weight for initial dosing calculations; use actual body weight of 35 kg unless specifically indicated (e.g., aminoglycosides). 1
• Avoid body surface area normalization for drugs that should be weight-based in this age group, particularly those metabolized by CYP2D6 or UGT enzymes. 3
• Maximum doses: Ensure pediatric weight-based calculations do not exceed standard adult maximum doses for any medication. 1