What intramuscular ceftriaxone dose is appropriate for a 3‑month‑old infant weighing 7.3 kg?

Ceftriaxone Intramuscular Dosing for a 3-Month-Old Infant (7.3 kg)

For a 3-month-old infant weighing 7.3 kg, administer ceftriaxone 50–75 mg/kg/day intramuscularly once daily (365–548 mg/day), with the specific dose determined by infection severity and type.

Standard Pediatric Dosing Algorithm

Non-Meningeal Infections (Most Common)

• For skin/soft tissue infections, pneumonia, sepsis, or other serious infections: administer 50–75 mg/kg/day given once daily or divided every 12 hours, with a maximum of 2 grams daily 1, 2.
• For this 7.3 kg infant: the calculated dose is 365–548 mg/day (50–75 mg/kg × 7.3 kg) 1.
• Practical dosing: round to 400–500 mg IM once daily for ease of administration 1.

Meningitis or CNS Infections

• For bacterial meningitis: the initial dose is 100 mg/kg/day (not to exceed 4 grams), which can be given once daily or divided every 12 hours 1, 2.
• For this 7.3 kg infant with meningitis: administer 730 mg/day (100 mg/kg × 7.3 kg), typically divided as 365 mg every 12 hours 1, 2.

Acute Otitis Media (Single-Dose Regimen)

• For treatment-refractory acute otitis media: a single IM dose of 50 mg/kg (maximum 1 gram) is recommended 2.
• For this 7.3 kg infant: administer 365 mg IM as a single dose 2.

Age-Specific Considerations for Young Infants

Neonatal Dosing (If <28 Days Old)

• The FDA label specifies that neonates >7 days old and >2000 g should receive 50–75 mg/kg/day every 24 hours 1.
• Critical contraindication: ceftriaxone is contraindicated in neonates ≤28 days who require calcium-containing IV solutions due to precipitation risk 2.
• Hyperbilirubinemia warning: ceftriaxone should not be used in hyperbilirubinemic neonates due to risk of bilirubin encephalopathy 1, 2.

Post-Neonatal Period (>28 Days, Which Applies Here)

• At 3 months of age, this infant is beyond the neonatal period and follows standard pediatric dosing of 50–75 mg/kg/day for non-meningeal infections 1, 2.

Intramuscular Administration Technique

Reconstitution for IM Injection

• For a 500 mg vial: add 1.8 mL diluent to achieve 250 mg/mL concentration, or add 1.0 mL diluent for 350 mg/mL concentration 2.
• For a 1 gram vial: add 3.6 mL diluent for 250 mg/mL, or 2.1 mL diluent for 350 mg/mL 2.

Injection Technique

• Inject deep into a large muscle mass (e.g., vastus lateralis in infants) with aspiration to avoid intravascular injection 2.
• Counsel caregivers that IM ceftriaxone is painful; consider lidocaine as diluent if not contraindicated 1, 3.

Indication-Specific Dosing Examples

Community-Acquired Pneumonia

• Standard dose: 50–100 mg/kg/day, with the higher range (80–100 mg/kg/day) preferred for hospitalized infants or suspected resistant S. pneumoniae 1.
• For this 7.3 kg infant: administer 365–730 mg/day (50–100 mg/kg), typically 500–600 mg IM once daily for moderate-to-severe pneumonia 1.

Sepsis or Bacteremia

• Standard dose: 50–75 mg/kg/day once daily 1, 2.
• For this 7.3 kg infant: administer 365–548 mg/day, rounded to 400–500 mg IM once daily 1.

Gonococcal Infections (Rare in This Age Group)

• For disseminated gonococcal infection or bacteremia: 50 mg/kg/day (maximum 1 gram) for 7 days 1, 3.
• For this 7.3 kg infant: administer 365 mg IM once daily for 7 days 1.

Treatment Duration

Standard Duration by Infection Type

• Most serious infections: continue for at least 2 days after clinical improvement, with typical total duration of 4–14 days depending on infection severity 2.
• Pneumonia: typically 7–10 days total 1, 4.
• Meningitis: 10–14 days for most bacterial causes 1, 2.
• Streptococcal infections: continue for at least 10 days to prevent rheumatic fever 2.

Common Pitfalls to Avoid

Underdosing Severe Infections

• Do not use the lower end (50 mg/kg/day) for severe sepsis, meningitis, or pneumonia with consolidation—always use 75–100 mg/kg/day for life-threatening infections 1.

Neonatal Contraindications

• Never administer ceftriaxone to hyperbilirubinemic neonates or those receiving calcium-containing IV solutions 1, 2.
• At 3 months, this infant is beyond the high-risk neonatal period, but always check bilirubin if jaundice is present 2.

Inadequate Muscle Mass

• In small infants, ensure injection into the vastus lateralis (anterolateral thigh), which has adequate muscle mass for IM administration 2.

Failure to Adjust for Meningitis

• If meningitis is suspected or confirmed, immediately increase to 100 mg/kg/day (730 mg/day for this infant) 1, 2.

Practical Ordering Example

For a 3-month-old, 7.3 kg infant with community-acquired pneumonia:

• Order: Ceftriaxone 500 mg IM once daily (approximately 68 mg/kg/day) for 7–10 days 1, 2.
• Reconstitution: Use 500 mg vial with 1.8 mL sterile water to achieve 250 mg/mL; draw up 2 mL for 500 mg dose 2.
• Administration: Inject deep into vastus lateralis muscle 2.

For the same infant with suspected meningitis:

• Order: Ceftriaxone 365 mg IM every 12 hours (total 730 mg/day = 100 mg/kg/day) for 10–14 days 1, 2.