When Fosfomycin Is Not Listed on Culture Sensitivity Reports
Use nitrofurantoin, trimethoprim-sulfamethoxazole, or amoxicillin-clavulanate as first-line alternatives for uncomplicated lower urinary tract infections when fosfomycin is not on the sensitivity report. These agents have equivalent clinical efficacy to fosfomycin and are recommended by major international guidelines 1.
Understanding Why Fosfomycin May Be Missing
Fosfomycin is often not routinely tested on standard culture sensitivity panels because:
- Many laboratories do not include fosfomycin in their standard antibiotic susceptibility testing panels 2
- This does not mean the organism is resistant—it simply wasn't tested 2
- Fosfomycin can be specifically requested if needed for multidrug-resistant organisms 3, 4
First-Line Alternative Antibiotics
For Uncomplicated Cystitis in Women
Nitrofurantoin is the preferred alternative:
- Dose: 100 mg twice daily for 5 days 1
- Equivalent clinical efficacy to fosfomycin (RR 1.01,95% CI 0.94-1.09) 1
- Minimal collateral damage to intestinal flora 3, 5
- Effective against 93-96% of E. coli isolates, including ESBL-producers 6
Trimethoprim-sulfamethoxazole (if local resistance <20%):
- Dose: 160/800 mg twice daily for 3 days 1
- Equivalent to fluoroquinolones for symptomatic cure (RR 1.00,95% CI 0.97-1.03) 1
- Critical caveat: Only use if local resistance rates are <20% for lower UTI 1
- Avoid if patient had recent exposure to this antibiotic 3
Amoxicillin-clavulanate:
- Recommended as an alternative first-choice option 1
- Particularly useful for young children 1
- Important limitation: Plain amoxicillin should be avoided due to 75% median resistance rates globally 1
Second-Line Options Based on Sensitivity Results
If Organism Shows Susceptibility
Oral cephalosporins:
- Cephalexin or cefixime can be used as second-line agents 3, 4
- Confirm susceptibility on culture report before prescribing 1
Fluoroquinolones (use sparingly):
- Ciprofloxacin or levofloxacin only if sensitivity confirmed 1, 3
- Major caveat: Fluoroquinolones should be restricted due to increasing resistance and antimicrobial stewardship concerns 1, 4
- Many countries no longer meet the <10% resistance threshold for empiric use 1
For Multidrug-Resistant Organisms
ESBL-Producing Enterobacteriaceae
If the culture shows ESBL production and fosfomycin isn't tested:
Oral options with high retained activity:
- Nitrofurantoin: >95% sensitivity to ESBL-producers 6
- Pivmecillinam (where available): >95% sensitivity to ESBL-producers 3, 6
- Amoxicillin-clavulanate: Can be effective for ESBL-E. coli (not Klebsiella) 3
Parenteral options for severe cases:
- Carbapenems (ertapenem preferred for single daily dosing) 1, 3
- Piperacillin-tazobactam for non-severe infections 1, 3
- Ceftazidime-avibactam or ceftolozane-tazobactam 3, 4
Clinical Decision Algorithm
Review the actual sensitivity report for nitrofurantoin, trimethoprim-sulfamethoxazole, and amoxicillin-clavulanate 1
For uncomplicated cystitis: Choose nitrofurantoin if sensitive (5-day course) 1, 5
If nitrofurantoin resistant: Use trimethoprim-sulfamethoxazole if sensitive and local resistance <20% 1
If both resistant: Consider amoxicillin-clavulanate or request fosfomycin susceptibility testing 1, 3
For ESBL organisms: Nitrofurantoin remains highly effective for uncomplicated UTI; reserve carbapenems for complicated infections or pyelonephritis 3, 6
Critical Pitfalls to Avoid
Do not use fluoroquinolones empirically without confirmed susceptibility—resistance rates have exceeded acceptable thresholds in many regions 1, 4
Do not use plain amoxicillin for UTI treatment due to 75% global resistance rates in E. coli 1
Do not use nitrofurantoin for pyelonephritis—it does not achieve adequate tissue concentrations outside the bladder 2, 5
Verify local resistance patterns before choosing trimethoprim-sulfamethoxazole, as many communities have exceeded the 20% resistance threshold 1, 3
When to Request Fosfomycin Testing
Consider specifically requesting fosfomycin susceptibility testing when: