How do you dilute gentamicin to 480 mL for Intramuscular (IM) injection?

Critical Error in Question: Gentamicin Cannot Be Diluted to 480 mL for IM Injection

You should NOT dilute gentamicin to 480 mL for intramuscular injection—this volume is far too large for IM administration and would be inappropriate and potentially dangerous.

Why This Request Is Problematic

Volume Constraints for IM Injection

• Maximum IM injection volume in adults is typically 3-5 mL per injection site, making a 480 mL IM injection physically impossible 1
• IM injections require concentrated formulations to minimize tissue trauma and ensure absorption 1
• The proposed dilution would create an impossibly large volume that cannot be administered intramuscularly

Standard Gentamicin Administration

• Gentamicin for systemic infections must be administered intravenously (IV) or intramuscularly (IM) in appropriate volumes, typically 1-3 mL per IM dose 2
• Standard dosing is 3 mg/kg/day IV or IM in divided doses (either once daily or in 2-3 equally divided doses depending on indication) 2
• For endocarditis treatment, gentamicin is dosed at 3 mg/kg per 24 hours IV/IM in 1-3 equally divided doses 2

Correct Approach to Gentamicin Preparation

For IM Administration

• Use the concentrated formulation (960 mg/3 mL) directly or dilute minimally to achieve 1-3 mL injection volume 1
• Calculate the required dose based on patient weight (3 mg/kg) 2
• Draw up the appropriate volume from the vial for immediate IM injection

For IV Administration

• Gentamicin can be diluted in 50-100 mL of normal saline or D5W for IV infusion over 30-60 minutes 1
• This is the only scenario where larger dilution volumes are appropriate, but still nowhere near 480 mL

Clinical Recommendation

If you need to administer gentamicin, determine the patient's weight, calculate the appropriate dose (3 mg/kg), and administer the concentrated formulation IM (1-3 mL) or dilute in 50-100 mL for IV infusion. The 480 mL dilution request suggests a fundamental misunderstanding of gentamicin administration that requires immediate clarification with pharmacy or infectious disease consultation 1, 3.