Cephalexin Dosing for UTI in a 70-Year-Old Woman
Recommended Dosing Regimen
For uncomplicated cystitis in this 70-year-old woman weighing 130 lb (59 kg), administer cephalexin 500 mg every 12 hours (twice daily) for 7 days. 1
Rationale for This Dosing Strategy
The FDA-approved dosing for uncomplicated cystitis in adults over 15 years is 500 mg every 12 hours, which provides adequate urinary concentrations (500-1000 mcg/mL) that far exceed the minimum inhibitory concentration for common uropathogens 1, 2
Recent evidence demonstrates that twice-daily dosing (500 mg BID) is equally effective as four-times-daily dosing (500 mg QID) for uncomplicated UTI, with no difference in treatment failure rates (12.7% vs 17%, p=0.343) 3
The twice-daily regimen improves patient adherence while maintaining clinical efficacy 3
Important Considerations for This Patient Population
Age-Related Renal Function Assessment
You must assess renal function before finalizing the dose, as patients over 70 years commonly have reduced creatinine clearance even with normal serum creatinine 2
If creatinine clearance is less than 30 mL/min, reduce the dosing frequency proportionally to the degree of renal impairment 2
For patients with significant renal dysfunction (CrCl <30 mL/min), consider extending the dosing interval to every 24 hours rather than every 12 hours 4, 2
Treatment Duration
Continue therapy for 7-14 days for uncomplicated cystitis 1
The standard 7-day course is appropriate for most cases of uncomplicated UTI in this age group 5
Clinical Context and Limitations
Position in Treatment Algorithm
Cephalexin is classified as an alternative agent rather than first-line therapy for uncomplicated UTI 5
β-lactams including cephalexin should be used when first-line agents (trimethoprim-sulfamethoxazole, nitrofurantoin, fosfomycin) cannot be used due to resistance patterns, allergies, or contraindications 5
β-lactams generally have inferior efficacy and more adverse effects compared to first-line UTI antimicrobials 5
Resistance Considerations
First-generation cephalosporins like cephalexin are less well-studied than other β-lactams for UTI treatment 5
Ensure local resistance patterns support cephalexin use, as it provides coverage primarily for E. coli, Klebsiella, and Proteus mirabilis 4, 2
Common Pitfalls to Avoid
Do not use cephalexin for complicated UTI or pyelonephritis - it does not achieve adequate tissue concentrations for parenchymal infections and is reserved for uncomplicated lower UTI only 5, 6
Do not assume normal renal function based on serum creatinine alone in elderly patients - calculate creatinine clearance to guide dosing adjustments 2
Do not use four-times-daily dosing when twice-daily dosing is equally effective and improves adherence 3
Avoid cephalexin if the patient has used it recently or if local E. coli resistance to first-generation cephalosporins exceeds 20% 5