What is the recommended IV doxycycline dose and infusion protocol for a 14‑year‑old child weighing 78 kg with normal hepatic and renal function and no hypersensitivity?

IV Doxycycline Dosing for a 14-Year-Old, 78 kg Patient

For this 14-year-old patient weighing 78 kg (>45 kg), administer 200 mg IV on the first day (given as 100 mg every 12 hours), followed by 100-200 mg daily depending on infection severity, with the standard adult dosing protocol applied. 1

Weight-Based Dosing Determination

• Since the patient weighs >45 kg (78 kg), the adult dosing regimen should be used rather than pediatric weight-based calculations 1, 2
• For children weighing ≥45 kg (100 pounds), the FDA-approved dosing is identical to adult dosing: 200 mg on day 1, then 100 mg daily maintenance 1
• The 2.2 mg/kg dosing used for children <45 kg does not apply to this patient 1, 2

IV Administration Protocol

Infusion Guidelines

• Infuse over 1-4 hours; for a 100 mg dose at 0.5 mg/mL concentration, the minimum infusion time is 1 hour 1
• Rapid administration must be avoided to prevent thrombophlebitis 1
• Prolonged IV therapy may result in thrombophlebitis; transition to oral therapy as soon as clinically appropriate 1

Preparation Instructions

• Reconstitute 100 mg vial with 10 mL Sterile Water for Injection to create 10 mg/mL concentration 1
• Further dilute each 100 mg in 100-1000 mL of compatible IV solution (0.9% NaCl, D5W, Lactated Ringer's, etc.) 1
• Never administer intramuscularly or subcutaneously; avoid inadvertent soft tissue infiltration 1

Condition-Specific Dosing Adjustments

• For severe/life-threatening infections (anthrax, Rocky Mountain Spotted Fever): Use 200 mg daily (100 mg every 12 hours) 1, 2
• For tickborne rickettsial diseases: 100 mg every 12 hours for minimum 5-7 days total, continuing at least 3 days after fever resolves 2, 3
• For anthrax post-exposure prophylaxis: 100 mg twice daily for 60 days total (transition to oral when feasible) 1, 2

Critical Administration Considerations

Renal Function

• No dose adjustment needed for renal impairment—doxycycline does not accumulate excessively in patients with renal dysfunction at recommended doses 1
• This is a key advantage over other tetracyclines 1

Transition to Oral Therapy

• Switch to oral doxycycline as soon as the patient can tolerate oral intake—parenteral therapy should not be prolonged unnecessarily 1
• When transitioning, maintain the same total daily dose (100-200 mg/day) 2, 3
• Oral bioavailability is approximately 90%, making the transition straightforward 4

Important Safety Precautions

Esophageal Protection

• Once transitioned to oral therapy, instruct the patient to take doxycycline with a full glass of water and remain upright for 30 minutes to prevent esophageal irritation and ulceration 3, 5

Drug Interactions

• Avoid co-administration with antacids containing aluminum, calcium, or magnesium within 2-3 hours of doxycycline administration 3
• Iron supplements and dairy products (if taking on empty stomach) should also be separated by 2-3 hours 3

Photosensitivity

• Counsel the patient to avoid extensive sun exposure or UV irradiation due to photosensitivity risk 5, 3

Common Pitfalls to Avoid

• Do not use pediatric weight-based dosing (2.2 mg/kg) for patients ≥45 kg—this would result in underdosing (171.6 mg for this 78 kg patient vs. the correct 200 mg) 1
• Do not infuse too rapidly—this increases thrombophlebitis risk 1
• Do not continue IV therapy longer than necessary—oral therapy has excellent bioavailability and should be instituted promptly 1, 4
• At age 14, tooth staining is not a concern as permanent dentition is complete 2