What is the recommended subcutaneous (SC) methotrexate dose for a 54 kg patient according to the British National Formulary for Children (BNFC) guidelines?

Subcutaneous Methotrexate Dosing for a 54 kg Patient According to BNFC

For a 54 kg patient, the recommended subcutaneous methotrexate dose according to the British National Formulary for Children (BNFC) is 10.8 mg weekly (calculated at 0.2 mg/kg body weight).

Dosing Calculation and Rationale

The dosing for methotrexate is weight-based according to guidelines:

• The maximum recommended dose should not exceed 0.2 mg/kg body weight 1
• For a 54 kg patient: 54 kg × 0.2 mg/kg = 10.8 mg weekly

Administration Considerations

• Subcutaneous administration offers several advantages over oral administration:

  • Better bioavailability
  • Higher efficacy at lower doses
  • Fewer gastrointestinal side effects 1, 2
  • More consistent absorption

• The dose should be administered once weekly on the same day each week to establish a routine and prevent dosing errors 3

Monitoring Requirements

Regular monitoring is essential when administering methotrexate:

• Initial monitoring: Blood tests every 7-14 days for the first month
• Ongoing monitoring: Blood tests every 2-3 months once therapy is stabilized 3
• Laboratory tests should be performed 4-6 days after dosing (not immediately after) due to transient elevations in hepatic transaminases 1

Important Safety Considerations

1. Folate supplementation: Must be prescribed alongside methotrexate to reduce toxicity

   • Recommended dose: 5 mg weekly or 1 mg daily (except on methotrexate day) 3
   • Reduces gastrointestinal side effects and early bone marrow toxicity 1

2. Contraindications:

   • Pregnancy or breastfeeding
   • Significant hepatic damage
   • Anemia, leucopenia, or thrombocytopenia 1

3. Drug interactions that increase methotrexate toxicity:

   • NSAIDs
   • Certain antibiotics (co-trimoxazole, trimethoprim)
   • Proton pump inhibitors
   • Salicylates
   • Alcohol 1, 3

Common Pitfalls to Avoid

1. Dosing frequency errors: Methotrexate must be taken ONCE WEEKLY, not daily, to prevent fatal toxicity 3

2. Inadequate monitoring: Failure to monitor blood counts and liver function can lead to undetected toxicity

3. Omitting folate supplementation: This significantly increases the risk of side effects

4. Alcohol consumption: Should be restricted to minimize hepatotoxicity risk 3

5. Timing of laboratory tests: Should be performed 4-6 days after dosing, not immediately after, to avoid misinterpreting transient elevations 1

For pediatric patients ages 13 years and older of average weight, adult dosing principles can be applied, with a maximum dose of 25 mg/week 1. However, for a 54 kg patient, the calculated dose of 10.8 mg weekly is appropriate to start with, and can be adjusted based on clinical response and tolerance.