Ampicillin Dosing Guidelines for Adults with Susceptible Infections
For adults with susceptible infections, ampicillin should be dosed at 2g IV every 4 hours (up to 12g daily) with appropriate dose adjustments for renal impairment. 1
Standard Dosing Regimens
- For uncomplicated urinary tract infections caused by susceptible organisms, ampicillin can be administered at 18-30g/day IV in divided doses 1
- For infective endocarditis, ampicillin should be administered at 2g IV every 4 hours (12g daily) 2, 1
- For HACEK group infections (when susceptibility is confirmed), ampicillin can be given at 2g IV every 4 hours 2, 1
- For enterococcal infections, ampicillin 2g IV every 4 hours is recommended as part of a combination therapy regimen 2
Renal Dosage Adjustments
- For patients with creatinine clearance <50 mL/min, a double β-lactam regimen (ampicillin plus ceftriaxone) is recommended over aminoglycoside combinations to avoid nephrotoxicity 2
- For patients with severe renal impairment (CrCl <10 mL/min), reduce dose to 1-2g every 6-8 hours and monitor drug levels if available 1
- In patients on continuous ambulatory peritoneal dialysis (CAPD), ampicillin should be administered every 12 hours rather than standard 4-6 hour intervals 3
Treatment Duration by Infection Type
- For uncomplicated UTIs: 3-7 days 1, 4
- For complicated UTIs: 7-14 days 4
- For infective endocarditis: 4-6 weeks 2, 1
- For native valve endocarditis with symptoms <3 months: 4 weeks 2
- For prosthetic valve endocarditis: minimum 6 weeks 2
Special Considerations
Combination Therapy Requirements
- For enterococcal endocarditis, ampicillin should be combined with gentamicin (3 mg/kg/day in divided doses) for synergistic effect 2
- For patients with renal impairment or risk of ototoxicity, ampicillin can be combined with ceftriaxone (2g IV every 12h) instead of aminoglycosides 2
- For vancomycin-resistant enterococci (VRE) causing uncomplicated UTIs, high-dose ampicillin (18-30g/day IV in divided doses) is recommended 2, 1
Susceptibility Considerations
- For HACEK microorganisms, ampicillin should only be used after confirming susceptibility, as β-lactamase production is increasingly common 2, 1
- For enterococci, ampicillin is the drug of choice when the organism is susceptible 4
- For β-lactamase-producing enterococci, ampicillin-sulbactam should be used instead of ampicillin alone 2
Monitoring and Safety
- Monitor renal function regularly during treatment, especially with high doses or prolonged therapy 2
- When using combination therapy with aminoglycosides, monitor for signs of nephrotoxicity and ototoxicity 2
- For patients receiving high-dose therapy (>12g/day), monitor serum levels if available to avoid toxicity 1
- Therapeutic drug monitoring is particularly important in patients with fluctuating renal function 3
Common Pitfalls and Caveats
- Ampicillin should not be used empirically for HACEK infections without susceptibility testing due to increasing β-lactamase production 2, 1
- For enterococcal infections, monotherapy with ampicillin is often insufficient for serious infections like endocarditis; combination therapy is required 2
- In patients with penicillin allergies, vancomycin is the alternative, but it's less effective than ampicillin for susceptible enterococci 2
- Sputum concentrations of ampicillin are approximately 30-40 times lower than corresponding serum levels, which may impact efficacy in respiratory infections 5