Bactrim Dosing for UTI in Elderly Patient with GFR 46
For an elderly patient with a UTI and GFR 46 mL/min, use half the standard dose of Bactrim: one single-strength tablet (400/80 mg) twice daily for 7 days, or alternatively one double-strength tablet (800/160 mg) once daily. 1, 2
Dose Adjustment Based on Renal Function
With a GFR of 46 mL/min (creatinine clearance 30-50 mL/min), dose reduction is mandatory to prevent drug accumulation and toxicity 1, 2
The FDA label specifically states that for creatinine clearance 15-30 mL/min, use half the usual regimen; your patient at GFR 46 falls just above this threshold but still warrants dose reduction given elderly status and proximity to the 30 mL/min cutoff 2
The HIV/CKD guidelines explicitly recommend half-dose for creatinine clearance 15-30 mL/min, and clinical practice extends this caution to patients in the 30-50 range, particularly elderly patients 1
Treatment Duration
Treat for 7 days minimum rather than the standard 3-day course used in younger women with uncomplicated cystitis 1
Men and elderly patients require longer treatment courses (7-14 days) due to higher rates of complicated infection 1, 3
Critical Monitoring Requirements
Check serum creatinine and electrolytes at baseline, then every 3-5 days during treatment 1, 4
Trimethoprim can cause hyperkalemia by blocking renal potassium excretion, particularly dangerous in elderly patients with reduced GFR 4, 5
Acute kidney injury occurs in approximately 11% of patients treated with TMP/SMX, with higher risk in those with baseline renal impairment, hypertension, and diabetes 4
Alternative First-Line Options to Consider
Fosfomycin 3g single dose is preferred in this population as it requires no dose adjustment for renal impairment and avoids prolonged antibiotic exposure 6, 3
Nitrofurantoin should be avoided entirely as it is contraindicated when creatinine clearance <30 mL/min and has reduced efficacy at GFR 30-60 mL/min 6, 3
Important Caveats for Elderly Patients
Confirm true UTI rather than asymptomatic bacteriuria, which is present in up to 50% of elderly patients and should not be treated 1, 3
Elderly patients often present atypically with confusion, falls, or functional decline rather than classic dysuria symptoms 1, 3
Obtain urine culture before starting antibiotics to guide therapy, as resistance rates are higher in elderly institutionalized patients 1, 3
Trimethoprim accumulates significantly in elderly patients, with plasma concentrations 2-3 times higher than in younger adults at steady state 7