Rabies Immune Globulin Dosing: Use Actual Body Weight
Yes, actual body weight is used when dosing rabies immune globulin, and your calculation of 2700 units (18 mL) for a 131.5 kg patient is correct. 1
Standard Dosing Protocol
The FDA-approved dosing for rabies immune globulin is 20 IU/kg of body weight (0.133 mL/kg), administered at the time of the first vaccine dose 1. This calculation is based on actual body weight, not ideal or adjusted body weight.
Your Patient's Dose Calculation
- Weight: 131.5 kg
- Dose: 131.5 kg × 20 IU/kg = 2,630 IU
- Volume: 131.5 kg × 0.133 mL/kg = 17.5 mL
Your calculation of 2700 units (18 mL) is appropriate and falls within acceptable range 1.
Administration Guidelines
Multiple vials are typically required for larger patients. The FDA label specifically provides an example: a 70 kg individual requires 9.31 mL, necessitating 4 full 2-mL vials plus a partial vial 1.
Critical Administration Steps
- Infiltrate wounds first: If anatomically feasible, use the full dose to thoroughly infiltrate the area around and into the wounds 1
- Remaining dose intramuscularly: Inject any remaining volume IM at a site distant from vaccine administration 1
- Never exceed recommended dose: Because rabies immune globulin may partially suppress active antibody production, do not give more than the recommended dose 1
Evidence Supporting Actual Body Weight Dosing
Clinical trials established 20 IU/kg as the optimal dose that provides maximum circulating antibody with minimum interference of active immunization 1. Studies demonstrated that this weight-based dosing achieves detectable antibody titers of ≥0.05 IU/mL in 60% of patients by day 3 1.
Real-World Adherence Data
A multi-hospital retrospective study of 246 patients found that 98% of patients received doses within 10% of the 20 IU/kg recommendation, demonstrating high adherence to actual body weight-based dosing in clinical practice 2.
Common Pitfalls to Avoid
Do not use ideal body weight or adjusted body weight for rabies immune globulin. Unlike some medications where obesity requires dose adjustment (such as IVIG, which uses ideal body weight in obese patients 3, 4), rabies immune globulin dosing is always based on actual body weight 1.
Do not underdose due to cost concerns. The FDA label explicitly states that "arbitrary capping based on drug procurement costs is unacceptable" 1. Suboptimal dosing could result in treatment failure and death from rabies.
Ensure adequate wound infiltration. Only 56% of eligible patients in one study received proper wound infiltration despite guidelines recommending it 2. This is a critical step that should not be omitted.
Timing Considerations
Administer within 7 days of first vaccine dose. All patients in clinical studies received rabies immune globulin within 7 days of vaccination with 100% adherence 2. Research suggests it can be administered up to 5 days after vaccine initiation without significant antibody suppression 5.
Special Circumstances
For extensive wounds in small children: You may need to dilute the rabies immune globulin in normal saline to provide adequate volume for infiltration of all wounds 5. However, always maintain the total calculated dose based on actual body weight.