What is the appropriate cefalexin (Keflex) dosing for an otherwise healthy patient in their late 80s with normal renal function?

Safe Cephalexin Dosing in Patients ≥80 Years Old

For otherwise healthy patients in their late 80s with normal renal function, standard adult cephalexin dosing of 250-500 mg every 6-12 hours (1-4 g daily total) is appropriate, but renal function must be objectively verified using estimated GFR rather than serum creatinine alone, as age-related muscle loss can mask significant renal impairment. 1

Critical Pre-Treatment Assessment

Verify renal function accurately before prescribing:

• Calculate estimated GFR (eGFR), not just serum creatinine, as elderly patients may have falsely reassuring creatinine levels due to reduced muscle mass despite 40% reduced renal function in a typical 70-80+ year-old 2
• Recognize that renal function declines approximately 1% per year after age 30-40, meaning octogenarians have substantially reduced clearance even with "normal" laboratory values 3
• If borderline renal function is suspected (eGFR 30-60 mL/min), obtain a 24-hour urine collection for accurate creatinine clearance to enable precise dose adjustment 4

Standard Dosing for Normal Renal Function

When eGFR >60 mL/min (truly normal function):

• 250 mg every 6 hours for mild-to-moderate infections (streptococcal pharyngitis, uncomplicated skin infections) 1
• 500 mg every 12 hours as an alternative for the same indications 1
• 500 mg every 6 hours for more severe infections or less susceptible organisms 1
• Maximum daily dose should not exceed 4 g; if higher doses are needed, switch to parenteral cephalosporins 1

Dose Adjustments for Renal Impairment

The FDA label does not provide specific geriatric dose reductions, but research demonstrates critical need for adjustment:

• Patients with creatinine clearance <30 mL/min require proportional dose reduction based on the degree of impairment 5
• In anephric patients, single 250-500 mg doses produce high, prolonged serum concentrations with peak levels persisting 6-12 hours due to delayed absorption and absent clearance 6
• For moderate impairment (CrCl 10-30 mL/min), reduce dose by 50% or double the dosing interval (e.g., 250 mg every 12 hours instead of every 6 hours) 3
• For severe impairment (CrCl <10 mL/min), reduce to 25-50% of normal dose 3

Practical Dosing Strategy for Octogenarians

Follow this algorithmic approach:

1. Obtain baseline eGFR using Cockcroft-Gault or CKD-EPI equation, accounting for actual body weight and age 2
2. If eGFR >60 mL/min: Use standard adult dosing (250-500 mg every 6-12 hours depending on infection severity) 1
3. If eGFR 30-60 mL/min: Start with 250 mg every 8-12 hours and monitor clinical response 5
4. If eGFR <30 mL/min: Start with 250 mg every 12-24 hours, or obtain nephrology consultation 3, 5
5. Ensure adequate hydration before and during therapy, as elderly patients are prone to volume depletion 2

Clinical Evidence in Elderly Populations

Limited but reassuring data exist for geriatric use:

• A study of hospitalized patients with average age 78 years demonstrated that cephalexin 125-250 mg once daily in the evening effectively treated chronic urinary tract infections, maintaining bacteriuria-free intervals averaging 92 days 7
• This suggests elderly patients may achieve therapeutic benefit with lower total daily doses than younger adults, likely due to reduced renal clearance 7
• Twice-daily dosing (1 g every 12 hours) showed equivalent efficacy to four-times-daily dosing in chronic UTI treatment, supporting simplified regimens for compliance 8

Absorption and Distribution Considerations

Age does not significantly impair cephalexin absorption:

• Cephalexin is completely absorbed from the upper intestine (not stomach), achieving peak levels within 1 hour in most patients 5
• However, delayed absorption (peak at 6-12 hours) occurred in some anephric patients, suggesting monitoring for delayed response in severe renal impairment 6
• 70-100% of the dose appears in urine within 6-8 hours, producing concentrations of 500-1000 mcg/mL after 250-500 mg doses—far exceeding MICs for common uropathogens 5
• Tissue distribution is rapid except to CSF and aqueous humor; low protein binding (minimal) reduces risk of drug interactions 5

Common Pitfalls to Avoid

Do not assume normal renal function based on:

• Serum creatinine alone—muscle wasting in elderly patients produces falsely low values 2
• Patient's self-reported "normal" kidney function—objective testing is mandatory 4

Do not use standard adult doses without verification when:

• Patient has diabetes, hypertension, or cardiovascular disease (high risk for occult renal disease) 3
• Patient takes other nephrotoxic medications or has history of acute kidney injury 3

Do not forget to:

• Adjust doses if renal function changes during treatment (dehydration, concurrent illness) 5
• Monitor for drug accumulation signs (altered mental status, seizures in severe cases) 3

Safety Profile in Elderly

Cephalexin has favorable characteristics for geriatric use:

• Does not disturb lower bowel flora (absorbed high in intestine) 5
• Very low incidence of allergic reactions due to chemical stability 5
• Does not penetrate host cells, contributing to low side-effect profile 5
• No significant drug-drug interactions via cytochrome P450 (unlike macrolides) 2
• Can be administered in high oral doses without gastrointestinal irritation 5