Keflex (Cephalexin) Dosing in Dialysis Patients
For patients on hemodialysis, administer cephalexin 250-500 mg after each dialysis session (typically 3 times weekly), as the drug is significantly removed during dialysis and requires post-dialysis dosing to maintain therapeutic levels. 1, 2
Pharmacokinetic Rationale
The dosing adjustment for cephalexin in dialysis patients is based on critical pharmacokinetic changes:
- Hemodialysis removes approximately 58% of serum cephalexin over a 6-hour dialysis session, making pre-dialysis dosing ineffective 1
- In anephric patients, the serum half-life extends dramatically from 1.1 hours (normal) to 5.1-5.7 hours during dialysis, and up to 6.1-18.1 hours in chronic renal failure patients not on dialysis 2, 3
- Despite prolonged half-life, antimicrobial activity remains adequate 8 hours post-injection even after dialysis 2
Specific Dosing Algorithm
Post-dialysis administration:
- Standard dose: 250-500 mg orally after each dialysis session 1, 4
- Timing: Administer immediately after dialysis completion to prevent premature drug removal 5
- Frequency: Three times weekly (matching typical hemodialysis schedule) 6
For patients with residual renal function (CrCl <30 ml/min but not yet on dialysis):
- Reduce dosing frequency proportional to creatinine clearance 4, 3
- The elimination rate constant correlates with creatinine clearance: Ke = 0.0766 + 0.0060 × CrCl 3
Urinary Tract Infection Coverage
Cephalexin maintains excellent urinary concentrations even in renal impairment:
- Urine concentrations of 500-1000 mcg/ml follow standard 250-500 mg doses, far exceeding MIC for common uropathogens (E. coli, Klebsiella, Proteus mirabilis) 1, 4
- This makes cephalexin particularly effective for UTI treatment in dialysis patients despite reduced renal clearance 1
Critical Safety Considerations
Absorption variability in anephric patients:
- While peak levels typically occur within 1 hour, delayed absorption can occur with peaks at 6-12 hours in some anephric patients 1
- Monitor clinical response, as absorption patterns may be unpredictable 1
Protein binding and tissue distribution:
- Low protein binding (minimal) reduces risk of drug accumulation 4
- Does not penetrate host cells, contributing to favorable safety profile 4
- No metabolism occurs; 70-100% excreted unchanged in urine over 6-8 hours in normal function 4
Comparison to Alternative Cephalosporins
Cefazolin (first-generation IV cephalosporin):
- Requires more complex dosing adjustments based on creatinine clearance 7
- Renal clearance approximately 40 ml/min in normal subjects 7
- May be preferred for serious infections requiring IV therapy, but requires individualized calculation 8, 7
Common Pitfalls to Avoid
- Never dose pre-dialysis: The drug will be removed before achieving therapeutic effect 1, 2
- Don't use standard renal dosing charts: These apply to non-dialysis CKD patients and will result in underdosing 6
- Avoid nephrotoxic alternatives: Unlike aminoglycosides (which cause nephrotoxicity in 4% of patients with 25% irreversibility), cephalexin has minimal nephrotoxic potential 9
- Don't assume consistent absorption: Monitor for delayed peaks in anephric patients 1