What is the recommended dose of Meropenem (generic name) for an adult patient with normal renal function and a Serratia marcescens infection?

Meropenem Dosing for Serratia marcescens Infections

For an adult patient with normal renal function and Serratia marcescens infection, administer meropenem 1 gram IV every 8 hours by extended infusion over 3 hours. 1, 2

Standard Dosing Regimen

The FDA-approved dosing for complicated infections is 1 gram every 8 hours, administered either as a 15-30 minute infusion or as a 3-5 minute bolus injection 2. However, extended infusion over 3 hours is strongly recommended to optimize pharmacodynamic targets, particularly for critically ill patients or when treating organisms with higher MIC values 1, 3.

Key Dosing Principles

• Maintain the full 1 gram dose per administration rather than reducing individual doses, as smaller doses may compromise efficacy 3
• The standard regimen provides adequate coverage for most Serratia marcescens infections, which are typically carbapenem-susceptible 4
• Meropenem demonstrates excellent activity against Enterobacteriaceae including Serratia species, with in vitro activity against aerobic gram-negative bacilli superior to imipenem 4

Extended Infusion Strategy

Extended infusion over 3 hours maximizes the time that drug concentrations remain above the MIC, which is the primary pharmacodynamic parameter associated with meropenem efficacy 1, 3, 5. This approach is particularly important because:

• Beta-lactam antibiotics like meropenem require plasma concentrations above the MIC for at least 70% of the dosing interval for optimal efficacy 1
• In critically ill patients or deep-seated infections, maintaining concentrations 4-6 times above the MIC increases success rates 1
• Extended infusion is specifically recommended when MIC values are ≥8 mg/L or for carbapenem-resistant organisms 1, 3

Clinical Evidence for Extended Infusion

A case report of Serratia marcescens meningitis demonstrated that prolonged 3-hour infusion of meropenem 2 grams every 8 hours achieved 100% time above MIC in both serum and CSF, resulting in successful clinical outcomes 5. While meningitis requires higher doses, this illustrates the pharmacodynamic advantage of extended infusion.

Site-Specific Considerations

Bloodstream Infections or Sepsis

• 1 gram IV every 8 hours by extended infusion 1
• Treatment duration: 7-14 days depending on source control and clinical response 1

Complicated Intra-Abdominal Infections

• 1 gram IV every 8 hours 1, 2
• Treatment duration: 5-7 days once adequate source control is achieved 1, 3

Pneumonia

• Consider 2 grams IV every 8 hours by extended infusion for severe cases 1
• Treatment duration: at least 7 days 1

Central Nervous System Infections

• 2 grams IV every 8 hours for meningitis or CNS involvement 1, 5
• Extended infusion over 3 hours is critical for CNS penetration 5

Renal Function Adjustments

For patients with impaired renal function, maintain the full 1 gram dose but extend the dosing interval rather than reducing individual doses 6, 3:

• CrCl 26-50 mL/min: 1 gram every 12 hours 2
• CrCl 10-25 mL/min: 500 mg every 12 hours 2
• CrCl <10 mL/min: 500 mg every 24 hours 2

For patients on continuous renal replacement therapy (CRRT), use 1 gram every 8-12 hours as CRRT removes 25-50% of meropenem 6.

Treatment Duration

5-7 days is appropriate for most Serratia marcescens infections once adequate source control is achieved 1, 3. Duration should be individualized based on:

• Clinical response and resolution of fever 1
• Adequacy of source control 1, 3
• Inflammatory marker trends 3
• Infection site (CNS infections require 21 days) 1

Critical Pitfalls to Avoid

• Never delay antibiotic administration waiting for culture results—start empiric therapy immediately 3
• Do not reduce individual doses below 1 gram for serious infections, even in renal impairment; instead extend the dosing interval 3
• Avoid standard 30-minute infusions when treating critically ill patients or resistant organisms—use extended 3-hour infusions 1, 3
• Do not use meropenem monotherapy for polymicrobial infections such as gangrenous bowel; combination therapy with anti-MRSA agents may be necessary 3
• Monitor for neurological toxicity if trough concentrations exceed 64 mg/L, particularly in patients with renal impairment 6, 3

Therapeutic Drug Monitoring

While not routinely required for standard infections, therapeutic drug monitoring should be considered for critically ill patients, those with renal impairment, or when treating resistant organisms 6, 7. Target trough concentrations should remain below 64 mg/L to prevent seizures 6.