What is the appropriate management for a 50-year-old patient with group B strep (Streptococcus agalactiae) in their urine, who has been started on Macrobid (nitrofurantoin) before culture results are available?

Management of Group B Streptococcus in Urine Started on Macrobid

Switch from nitrofurantoin (Macrobid) to penicillin G 500 mg orally every 6-8 hours or ampicillin 500 mg orally every 8 hours for 7-10 days, as nitrofurantoin has poor activity against Group B Streptococcus and penicillin remains the preferred narrow-spectrum agent. 1

Critical First Step: Determine Pregnancy Status

• If pregnant: Any GBS bacteriuria during pregnancy—regardless of colony count or symptoms—mandates immediate treatment AND intrapartum antibiotic prophylaxis during labor to prevent neonatal disease 2, 3
• If not pregnant: Treatment depends entirely on whether the patient is symptomatic or has abnormal urinalysis findings 1, 2

Assessment of Clinical Significance in Non-Pregnant Patients

Symptomatic UTI (dysuria, frequency, urgency, suprapubic pain)

• Treat with appropriate antibiotics as this represents true infection requiring therapy 1
• Abnormal urinalysis findings (pyuria, positive leukocyte esterase, bacteria) support true infection rather than colonization 1

Asymptomatic Bacteriuria

• Do not treat if the patient has no genitourinary symptoms and normal urinalysis, as this represents asymptomatic bacteriuria that should not be treated 1, 4
• The 2019 IDSA guidelines provide strong evidence against treating asymptomatic bacteriuria in non-pregnant populations, including those with diabetes, long-term care residents, and catheterized patients 1
• Treating asymptomatic bacteriuria leads to unnecessary antibiotic exposure, resistance development, and potential adverse effects without clinical benefit 1

Antibiotic Selection for Confirmed GBS UTI

First-Line Therapy

• Penicillin G 500 mg orally every 6-8 hours for 7-10 days is preferred due to narrow spectrum of activity 1
• Ampicillin 500 mg orally every 8 hours for 7-10 days is an acceptable alternative 1
• All GBS isolates remain universally susceptible to penicillin, ampicillin, cephalosporins, and vancomycin 5

Penicillin-Allergic Patients

• Clindamycin 300-450 mg orally every 8 hours with susceptibility testing performed before use due to increasing resistance (up to 18-20% resistance rates) 1, 2, 5
• First-generation cephalosporins are acceptable if no immediate-type hypersensitivity to β-lactams 4

Complicated UTI or Severe Presentations

• Consider initial IV ampicillin 2 g every 4-6 hours, then transition to oral therapy once clinically stable 1
• Extend treatment to 14 days for complicated infections or when prostatitis cannot be excluded in men 1
• Combination therapy with ampicillin plus aminoglycoside may be warranted for severe presentations 1

Why Nitrofurantoin (Macrobid) is Inappropriate

• Nitrofurantoin has inadequate activity against Group B Streptococcus and should not be used for GBS UTI 1
• The empiric choice of Macrobid before culture results is reasonable for typical UTI pathogens (E. coli), but must be changed once GBS is identified 2

Common Pitfalls to Avoid

• Contamination vs. True Infection: Active menstruation is a well-recognized cause of false-positive urine cultures for GBS, which colonizes the vaginal tract 2
• If urinalysis is negative (except for blood) in a menstruating patient, consider repeating the culture after menses to distinguish contamination from true infection 2
• Do not treat asymptomatic GBS vaginal colonization outside the intrapartum period in pregnancy, as this is ineffective at eliminating carriage and promotes resistance 2
• Laboratory reporting: GBS at concentrations ≥10^4 CFU/ml represents clinically significant bacteriuria requiring treatment in symptomatic patients 6

Follow-Up Considerations

• Follow-up urine culture after treatment completion may be warranted to ensure eradication, especially in patients with recurrent UTIs 1
• In pregnancy: Women with documented GBS bacteriuria should not be re-screened in the third trimester, as they are presumed to remain colonized and require intrapartum prophylaxis 3
• Antibiotic susceptibility testing should be performed for penicillin-allergic patients to guide alternative therapy 6