Cefazolin (Ancef) for Streptococcus agalactiae UTIs
Cefazolin is an effective and appropriate treatment option for Streptococcus agalactiae (Group B Streptococcus) urinary tract infections, with documented in vitro activity and clinical efficacy, though beta-lactams with narrower spectrum or better GBS activity (such as ampicillin or amoxicillin) are generally preferred first-line agents.
Microbiological Activity
- Cefazolin demonstrates documented in vitro activity against Streptococcus agalactiae, as confirmed by FDA labeling which lists GBS among susceptible organisms 1
- However, cephalexin (a related first-generation cephalosporin) shows MICs of 2 to >16 mg/L against GBS strains, which is notably higher than ampicillin (0.1 to >1 mg/L) or amoxicillin (0.03 to 0.5 mg/L) 2
- Amoxicillin demonstrates 2.5 times greater activity than ampicillin against GBS, making penicillins generally more potent than first-generation cephalosporins for this pathogen 2
Clinical Evidence for UTI Treatment
- A 1977 study treating 45 UTI cases with cefazolin achieved cure in 77.77% overall, with particularly strong results (92.85% cure rate) in acute cystitis using 250 mg IM every 12 hours for 6-10 days 3
- For pyelonephritis, cefazolin 500 mg every 12 hours for 10-14 days achieved only 52.94% cure rate, suggesting it may be less optimal for upper tract infections 3
- A comparative study found cefazolin 1 gram IM every 8 hours equally effective as cefuroxime for complicated UTIs, with 97% negative cultures at day 1 and 63% sustained cure at one week post-treatment 4
Practical Recommendations
For Uncomplicated Cystitis:
- Cefazolin 250-500 mg IM/IV every 12 hours for 6-10 days is effective 3
- Consider oral alternatives (amoxicillin, cephalexin) when possible for outpatient management
For Complicated UTI or Pyelonephritis:
- Cefazolin 1 gram IV every 8 hours can be used, though cure rates for upper tract infections are suboptimal 3, 4
- Ampicillin or amoxicillin may be preferable given superior in vitro activity 2
For Penicillin-Allergic Patients:
- Cefazolin 2 g IV initial dose, then 1 g IV every 8 hours is recommended for patients without high-risk penicillin allergy (no history of anaphylaxis, angioedema, or urticaria) 5
- This recommendation comes from CDC guidelines for GBS prophylaxis but applies to treatment scenarios
Important Caveats
- All GBS isolates tested show universal susceptibility to penicillin, cefuroxime, cefaclor, and ceftriaxone, so resistance is not a concern 6
- Cefazolin should not be used for multidrug-resistant organisms or ESBL-producing pathogens that may co-exist in complicated UTIs 7
- Local susceptibility patterns should always guide final therapy selection 7
- The drug is well-tolerated with minimal adverse effects reported in clinical studies 3, 4