Recommended Antibiotic Coverage for Nitrite Positive Urinary Tract Infections
For a nitrite positive urine test indicating a urinary tract infection (UTI), first-line treatment should be nitrofurantoin (100 mg twice daily for 5 days) for uncomplicated cystitis in women, while trimethoprim-sulfamethoxazole (160/800 mg twice daily for 3-7 days) is recommended for men or as an alternative in women. 1
Antibiotic Selection Algorithm Based on Patient Factors
For Women with Uncomplicated Cystitis:
First-line options:
- Nitrofurantoin macrocrystals: 50-100 mg four times daily for 5 days
- Nitrofurantoin monohydrate/macrocrystals: 100 mg twice daily for 5 days
- Fosfomycin trometamol: 3 g single dose
- Pivmecillinam: 400 mg three times daily for 3-5 days
Alternative options (when first-line agents cannot be used):
- Trimethoprim-sulfamethoxazole: 160/800 mg twice daily for 3 days (avoid in last trimester of pregnancy)
- Trimethoprim: 200 mg twice daily for 5 days (avoid in first trimester of pregnancy)
- Cephalosporins (e.g., cefadroxil): 500 mg twice daily for 3 days (if local E. coli resistance <20%)
For Men with UTI:
- Recommended treatment:
- Trimethoprim-sulfamethoxazole: 160/800 mg twice daily for 7 days
- Fluoroquinolones (e.g., ciprofloxacin): According to local susceptibility testing
Clinical Considerations for Nitrite Positive UTIs
Significance of Nitrite Positive Test:
- A positive nitrite test has high specificity (94-100%) but moderate sensitivity (30.8-66.2%) for UTI 2, 3
- Positive predictive value ranges from 78.2-96% 2, 4
- A positive nitrite test strongly suggests the presence of nitrate-reducing bacteria (typically Enterobacteriaceae like E. coli, Klebsiella, Proteus)
Common Causative Organisms:
- Escherichia coli (most common, 56.9-74.9%)
- Klebsiella species (7.9-18.5%)
- Proteus species (5.6%)
- Enterococcus species (3.7-7.7%) - note that Enterococcus cannot reduce nitrates and will not produce a positive nitrite test 5, 3
Important Caveats:
- Do not adjust antibiotic choice based solely on nitrite results - studies show no significant correlation between nitrite positivity and antibiotic resistance patterns for most antibiotics 5, 6
- A negative nitrite test does not rule out UTI, as approximately 50% of culture-positive samples may have negative nitrite tests 2
- For complicated UTIs or pyelonephritis, broader coverage may be needed (see below)
Special Situations
Uncomplicated Pyelonephritis:
Outpatient oral therapy:
- Ciprofloxacin: 500-750 mg twice daily for 7 days
- Trimethoprim-sulfamethoxazole: 160/800 mg twice daily for 14 days
- Cefpodoxime: 200 mg twice daily for 10 days
Inpatient IV therapy:
- Fluoroquinolones, aminoglycosides, or extended-spectrum cephalosporins
- Consider broader coverage only if multidrug-resistant organisms are suspected
Complicated UTIs:
- Consider broader spectrum antibiotics based on risk factors for resistant organisms
- Obtain urine culture before starting antibiotics
- Treat for 7-14 days (14 days for men when prostatitis cannot be excluded)
- Address any underlying anatomical or functional abnormalities
Practical Recommendations
- Always obtain a urine culture before starting antibiotics in complicated cases, pyelonephritis, or recurrent UTIs
- Consider local resistance patterns when selecting empiric therapy
- For uncomplicated cystitis in women, nitrofurantoin remains an excellent first choice due to low resistance rates
- Trimethoprim-sulfamethoxazole should be used with caution in areas where resistance exceeds 20%
- Fluoroquinolones should be reserved for more serious infections due to concerns about resistance and adverse effects
Remember that while nitrite positivity confirms the presence of bacteria, it should not be the sole determinant of antibiotic selection. The choice of antibiotic should be based on patient factors, infection severity, and local resistance patterns.