I have a patient with an uncomplicated urinary tract infection caused by Escherichia coli; the susceptibility panel shows susceptibility to amoxicillin‑clavulanate (≤2 µg/mL), ampicillin‑sulbactam (≤2 µg/mL), cefazolin not reported, cefepime (≤0.12 µg/mL), ceftazidime (≤0.5 µg/mL), ceftriaxone (≤0.25 µg/mL), ciprofloxacin (≤0.06 µg/mL), gentamicin (≤1 µg/mL), imipenem (≤0.25 µg/mL), levofloxacin (≤0.12 µg/mL), meropenem (≤0.25 µg/mL), nitrofurantoin (≤16 µg/mL), piperacillin‑tazobactam (≤4 µg/mL), and trimethoprim‑sulfamethoxazole (≤20 µg/mL). How should I interpret this susceptibility panel and which oral antibiotic is appropriate for treatment?

Understanding Your E. coli UTI Susceptibility Panel

Your E. coli is susceptible to essentially all tested antibiotics, making this an uncomplicated UTI with excellent treatment options—prioritize nitrofurantoin, trimethoprim-sulfamethoxazole, or a fluoroquinolone based on local resistance patterns and patient factors.

Interpreting the Susceptibility Panel

What "S" Means

• "S" = Susceptible: The bacteria will be killed or inhibited by standard doses of that antibiotic 1
• The numbers (e.g., ≤2, ≤0.12) represent the Minimum Inhibitory Concentration (MIC) in µg/mL—the lowest concentration needed to stop bacterial growth 1
• Lower MIC values indicate the bacteria is more sensitive to that antibiotic 1

What "NR" Means for Cefazolin

• "NR" = Not Reported: The laboratory chose not to report this result, likely because cefazolin is not recommended for UTI treatment despite technical susceptibility 1
• This is a clinical decision by the lab to guide appropriate prescribing 1

Your Organism's Profile

• This E. coli strain shows pan-susceptibility (sensitive to all reported antibiotics) 1
• The extremely low MIC values (especially ciprofloxacin ≤0.06, levofloxacin ≤0.12) indicate a highly susceptible, non-resistant strain 1
• This is not an ESBL-producing or multidrug-resistant organism 2

Recommended Oral Treatment Options for Uncomplicated UTI

First-Line Choices (Preferred)

For uncomplicated cystitis:

• Nitrofurantoin: 100 mg twice daily for 5 days 1, 3, 2

  • Excellent choice with minimal collateral damage (doesn't promote resistance to other antibiotics) 4, 5
  • MIC ≤16 µg/mL confirms susceptibility 1

• Trimethoprim-sulfamethoxazole: 160/800 mg (one double-strength tablet) twice daily for 3 days 1

  • Your isolate shows MIC ≤20 µg/mL, confirming susceptibility 1
  • Only use if local E. coli resistance rates are <20% 4, 5

• Fosfomycin-trometamol: 3 g single oral dose 1, 3, 2

  • Not on your panel but would be effective given overall susceptibility pattern 4, 5

Second-Line Choices (Highly Effective but Reserve When Possible)

Fluoroquinolones (use only if first-line agents contraindicated or local resistance <10%):

• Ciprofloxacin: 250-500 mg twice daily for 3 days (cystitis) 1, 6

  • Your MIC ≤0.06 µg/mL indicates excellent susceptibility 1

• Levofloxacin: 250 mg once daily for 3 days (cystitis) 1, 6

  • Your MIC ≤0.12 µg/mL confirms susceptibility 1, 6

Important caveat: Fluoroquinolones cause significant "collateral damage" by selecting for multidrug-resistant organisms and should be reserved for complicated infections or pyelonephritis 1, 4, 5

Third-Line Options (Less Preferred)

Beta-lactams (less effective than other options):

• Amoxicillin-clavulanate: 875 mg twice daily for 5-7 days 1, 2
  • Your MIC ≤2 µg/mL confirms susceptibility 1
  • Never use plain amoxicillin—75% median resistance rate globally 3
  • Less effective than trimethoprim-sulfamethoxazole or fluoroquinolones for UTI 1, 7

Treatment Duration by Infection Type

Uncomplicated Cystitis (Lower UTI)

• Nitrofurantoin: 5 days 1, 2
• Trimethoprim-sulfamethoxazole: 3 days 1
• Fluoroquinolones: 3 days 1
• Beta-lactams: 5-7 days 1

Uncomplicated Pyelonephritis (Upper UTI)

• Fluoroquinolones: 5-7 days (ciprofloxacin 500 mg twice daily for 7 days OR levofloxacin 750 mg daily for 5 days) 1, 6
• Trimethoprim-sulfamethoxazole: 14 days (only if susceptible) 1
• Beta-lactams: 10-14 days (less effective, consider initial IV dose of ceftriaxone 1 g) 1

Clinical Decision Algorithm

Step 1: Confirm this is uncomplicated UTI (non-pregnant, no urologic abnormalities, no recent instrumentation) 1

Step 2: Determine infection severity:

• Lower UTI symptoms only (dysuria, frequency, urgency): Treat as cystitis 1
• Fever, flank pain, systemic symptoms: Treat as pyelonephritis 1

Step 3: Select antibiotic based on hierarchy:

1. First choice: Nitrofurantoin (if cystitis only—does not achieve adequate tissue levels for pyelonephritis) 1, 2
2. Second choice: Trimethoprim-sulfamethoxazole (if local resistance <20%) 1, 4
3. For pyelonephritis: High-dose fluoroquinolone (levofloxacin 750 mg daily preferred) 1, 6
4. If above contraindicated: Amoxicillin-clavulanate with longer duration 1

Critical Pitfalls to Avoid

• Do not use plain amoxicillin or ampicillin despite susceptibility on panel—clinical resistance rates are 75% and outcomes are poor 3
• Do not use cefazolin for UTI treatment (hence "NR" on your panel) 1
• Do not use nitrofurantoin for pyelonephritis—inadequate tissue penetration despite urine concentrations 1
• Avoid fluoroquinolones for simple cystitis when first-line agents are suitable—reserve for complicated infections to minimize resistance development 1, 4, 5
• Do not treat asymptomatic bacteriuria unless patient is pregnant or undergoing urologic procedures 1, 4
• Carbapenems, cefepime, ceftazidime, and piperacillin-tazobactam on your panel are IV agents reserved for complicated/hospital-acquired infections—not appropriate for uncomplicated outpatient UTI 1, 2

Why This Organism is Favorable

Your E. coli demonstrates:

• No ESBL production (ceftriaxone MIC ≤0.25 µg/mL) 1, 2
• No carbapenem resistance (meropenem/imipenem MICs ≤0.25 µg/mL) 1, 2
• No fluoroquinolone resistance (ciprofloxacin MIC ≤0.06 µg/mL) 1
• Community-acquired susceptibility pattern with excellent treatment options 1, 2