What pre‑operative gentamicin dose should be given to a 41‑year‑old female weighing 135 kg undergoing laparoscopic ovarian cystectomy?

Pre-operative Gentamicin Dosing for Laparoscopic Ovarian Cystectomy

For this 135 kg, 41-year-old female undergoing laparoscopic ovarian cystectomy, administer gentamicin 5 mg/kg IV as a single dose 30-60 minutes before surgical incision, which equals 675 mg (based on actual body weight), combined with either cefazolin 2g IV or clindamycin 900 mg IV. 1

Dosing Algorithm

Step 1: Calculate Gentamicin Dose Based on Actual Body Weight

• Use 5 mg/kg dosing based on actual body weight for surgical prophylaxis 1
• For this 135 kg patient: 5 mg/kg × 135 kg = 675 mg IV single dose 1
• The FDA label confirms 5 mg/kg/day dosing for life-threatening infections, and surgical prophylaxis guidelines specifically recommend this dose for pre-operative use 2, 1
• Administer 30-60 minutes before surgical incision to ensure adequate tissue concentrations at the time of incision 1

Step 2: Combine with Appropriate Beta-Lactam or Alternative

• Primary regimen: Gentamicin 675 mg IV + Cefazolin 2g IV (single dose, given 30-60 minutes pre-operatively) 1
• Beta-lactam allergy alternative: Gentamicin 675 mg IV + Clindamycin 900 mg IV (single dose) 1
• For gynecologic procedures including laparoscopic ovarian surgery, combination therapy targets both aerobic gram-negative organisms (E. coli, Enterobacteriaceae) and gram-positive cocci (Staphylococcus, Enterococcus) 1

Step 3: Timing and Re-dosing Considerations

• No re-dosing of gentamicin is required even if surgery exceeds expected duration, as single-dose prophylaxis is standard 1
• If cefazolin is used and surgery duration exceeds 4 hours, re-inject cefazolin 1g IV (but not gentamicin) 1
• Do not continue antibiotics post-operatively unless there is documented infection or contamination requiring therapeutic treatment 1, 3

Evidence Quality and Rationale

The 2019 French Society of Anesthesiology guidelines explicitly recommend gentamicin 5 mg/kg/day as a single dose for gynecologic procedures when combined with clindamycin for beta-lactam allergic patients 1. The 2008 American Urological Association guidelines similarly specify 5 mg/kg IV single dose for surgical prophylaxis 1. These guidelines do not recommend dose adjustment or capping for obesity – the dose should be calculated on actual body weight 1.

Research evidence supports that higher gentamicin doses (5-7 mg/kg) are necessary to achieve adequate peak concentrations, particularly in surgical patients with significant fluid shifts and blood loss 4, 5. A 2004 study demonstrated that standard 2 mg/kg dosing resulted in inadequate serum and tissue concentrations during colorectal surgery with significant fluid replacement 5. A 2021 study found that 89% of urologic surgery patients received guideline-discordant (lower than recommended) gentamicin doses, with no increased nephrotoxicity risk at higher doses 6.

Critical Pitfalls to Avoid

• Do not use lean body weight or adjusted body weight calculations – surgical prophylaxis guidelines specify actual body weight for aminoglycoside dosing in obese patients 1, 2
• Do not cap the gentamicin dose at arbitrary limits (e.g., 240 mg or 320 mg) – this results in subtherapeutic concentrations 6
• Do not administer gentamicin after surgical incision – efficacy depends entirely on achieving adequate tissue concentrations before bacterial contamination occurs 1
• Do not continue gentamicin post-operatively – single-dose prophylaxis is sufficient and continuation increases nephrotoxicity risk without benefit 1
• Do not use fluoroquinolones as monotherapy for gynecologic surgical prophylaxis – they are not recommended for this indication 1

Target Organisms

• Gram-negative aerobic bacteria: E. coli, Klebsiella, Proteus, other Enterobacteriaceae 1
• Gram-positive cocci: Staphylococcus aureus, Enterococcus species 1
• Anaerobic coverage (if using clindamycin or metronidazole in combination) 1