What is the recommended dose of Keflex (cephalexin) for a complicated urinary tract infection (UTI) in a man with impaired renal function?

Cephalexin Dosing for Complicated UTI in Men with Impaired Renal Function

For men with complicated UTI and impaired renal function, cephalexin dosing should be reduced proportionally to the degree of renal impairment, with 500 mg twice daily for mild impairment (CrCl 30-50 ml/min) and 250-500 mg once daily for moderate to severe impairment (CrCl <30 ml/min).

Assessment of Renal Function

First, determine the severity of renal impairment:

• Calculate creatinine clearance (CrCl) using the Cockcroft-Gault formula
• Categorize renal function:
  • Mild impairment: CrCl 30-50 ml/min
  • Moderate impairment: CrCl 10-30 ml/min
  • Severe impairment: CrCl <10 ml/min

Dosing Algorithm for Cephalexin in Complicated UTI

Normal Renal Function

• Standard dose: 500 mg orally four times daily for 14 days 1
• Alternative regimen: 1 g twice daily for 14 days 2

Impaired Renal Function

• Mild impairment (CrCl 30-50 ml/min): 500 mg twice daily for 14 days
• Moderate impairment (CrCl 10-30 ml/min): 250-500 mg once daily for 14 days
• Severe impairment (CrCl <10 ml/min): 250 mg once daily for 14 days 3

Treatment Duration

• 14 days is recommended for men with complicated UTI as prostatitis cannot be excluded 1
• Do not shorten treatment duration even if the patient becomes afebrile after 48 hours, as this may lead to treatment failure in men with complicated UTI

Monitoring Recommendations

• Obtain urine culture and susceptibility testing before initiating therapy
• Monitor renal function during treatment
• Assess clinical response after 48-72 hours
• Consider prothrombin time monitoring in patients with renal impairment as cephalosporins may affect prothrombin activity 4

Important Considerations

1. Microbial spectrum: Complicated UTIs have a broader microbial spectrum than uncomplicated UTIs, with higher likelihood of antimicrobial resistance 1. Common pathogens include E. coli, Proteus spp., Klebsiella spp., Pseudomonas spp., and Enterococcus spp.

2. Pharmacokinetics in renal impairment: Cephalexin is primarily eliminated by the kidneys, with 70-100% of the dose found in urine within 6-8 hours. Patients with CrCl <30 ml/min require dose reduction proportional to their reduced renal function 3.

3. Urinary concentrations: Even in patients with impaired renal function, urinary concentrations of cephalexin are typically adequate for treating most UTIs caused by E. coli, Klebsiella, and Proteus mirabilis 5.

4. Hemodialysis: Hemodialysis reduces serum cephalexin concentration by approximately 58% over 6 hours 5. Consider supplemental dosing after dialysis.

Pitfalls and Caveats

• Do not use cephalexin empirically without culture and susceptibility testing in complicated UTIs due to higher risk of resistant organisms 1
• Address any underlying urological abnormalities or complicating factors as part of management 1
• Monitor for superinfection with prolonged use 4
• Consider alternative agents if the patient has a history of immediate hypersensitivity reaction to penicillins due to potential cross-reactivity
• Cephalexin may not be effective against ESBL-producing organisms, which are considered a complicating factor in UTIs 1

By following this algorithm, you can appropriately dose cephalexin for men with complicated UTI and impaired renal function, maximizing efficacy while minimizing toxicity.