What is the recommended ampicillin dosage for a healthy adult, for children, and for patients with impaired renal function?

Ampicillin Dosing Recommendations

Healthy Adults

For healthy adults with normal renal function, ampicillin should be dosed at 2 g every 6 hours (8 g/day total) for serious infections, administered intravenously in divided doses. 1

• For intra-abdominal infections in critically ill patients at high risk for enterococcal infection, ampicillin 2 g every 6 hours should be added to the antimicrobial regimen if not already using piperacillin-tazobactam or imipenem-cilastatin (which have intrinsic enterococcal activity). 1

• For endocarditis caused by HACEK microorganisms, ampicillin-sulbactam 12 g per 24 hours IV in 4 equally divided doses (3 g every 6 hours) is recommended for 4 weeks in native valve endocarditis or 6 weeks in prosthetic valve endocarditis. 1

• For healthcare-associated intra-abdominal infections in non-critically ill patients at higher risk for multidrug-resistant organisms, ampicillin 2 g every 6 hours should be combined with meropenem or doripenem. 1

Pediatric Patients

For children with normal renal function, ampicillin should be dosed at 200 mg/kg/day IV divided every 6 hours, not to exceed adult dosing. 1

Age-Specific Pediatric Dosing:

• Neonates ≤7 days old and ≤2000 g: 50 mg/kg/day divided every 12 hours 1

• Neonates ≤7 days old and >2000 g: 75 mg/kg/day divided every 8 hours 1

• Neonates >7 days old and <1200 g: 50 mg/kg/day divided every 12 hours 1

• Neonates >7 days old and 1200-2000 g: 75 mg/kg/day divided every 8 hours 1

• Neonates >7 days old and >2000 g: 100 mg/kg/day divided every 6 hours 1

• Infants and children >1 month: 100-200 mg/kg/day divided every 6 hours 1

For ampicillin-sulbactam in pediatric patients, the dosing is 200 mg/kg/day of the ampicillin component divided every 6 hours for complicated intra-abdominal infections. 1

Patients with Impaired Renal Function

Ampicillin dosing must be adjusted based on creatinine clearance, as renal impairment significantly prolongs the half-life and reduces drug clearance. 2, 3

Renal Dosing Algorithm:

• CrCl >60 mL/min (normal function): Standard dosing of 2 g every 6 hours 2

• CrCl 31-60 mL/min (mild renal failure): Standard dosing can be maintained, though monitoring is advisable 2

• CrCl 7-30 mL/min (severe renal failure): Reduce frequency to 2 g every 12 hours (twice daily), as the half-life more than doubles in this population 2

• CrCl <7 mL/min (end-stage renal disease): Administer 2 g every 24 hours (once daily) 2

Hemodialysis Considerations:

• Ampicillin is significantly removed by hemodialysis, with approximately 35% of the dose cleared during a 4-hour treatment session. 2

• Dosing on hemodialysis days: Administer the dose after hemodialysis is completed to avoid premature drug removal. 2

• The half-life during hemodialysis decreases to approximately 2.2 hours, compared to 17.4 hours between dialysis sessions. 2

• A slight rebound in serum concentrations occurs after hemodialysis due to redistribution from tissues. 2

Critical Considerations for Renal Impairment:

The nearly parallel decrease in total body clearance with relatively constant volume of distribution and nonrenal clearance suggests that the same ratio of ampicillin to sulbactam (2:1) is appropriate regardless of renal function. 2

For critically ill patients with augmented renal clearance (eGFR >130 mL/min), standard doses may be insufficient, and higher doses or continuous infusion should be considered with therapeutic drug monitoring. 4

Common Pitfalls to Avoid:

• Do not use first-generation cephalosporins as substitutes for ampicillin in serious infections, as they lack activity against enterococci and many gram-negative pathogens. 1

• Avoid underdosing in patients with normal or augmented renal function, particularly in critically ill patients who may require 50% higher doses to achieve therapeutic levels. 4

• Never administer hemodialysis doses before dialysis, as this results in subtherapeutic levels throughout the interdialytic period. 2

• Do not assume urinary tract infections in severe renal failure require dose reduction for efficacy—urine concentrations of 88.6 mcg/mL are achievable even with standard dosing and remain well above MIC values. 5