Is Enterococcus faecium in a urine culture considered a contaminant?

Is Enterococcus faecium in Urine Culture a Contaminant?

Enterococcus faecium in urine culture is generally not considered a contaminant but rather a true pathogen that requires clinical evaluation and potential treatment, especially in symptomatic patients or those undergoing urologic procedures. 1

Understanding Enterococcus faecium in Urine Cultures

Enterococci, particularly E. faecium, have emerged as important urinary tract pathogens, especially in hospitalized patients. Unlike typical contaminants, E. faecium possesses several characteristics that make it a significant clinical concern:

• E. faecium is characterized by natural resistance to numerous antibiotics (including cephalosporins)
• It has a high capacity for acquiring additional resistance mechanisms
• It can cause persistent and difficult-to-treat infections 2

Distinguishing True Infection from Contamination

To determine whether E. faecium represents true infection or contamination, consider:

1. Collection method: The method used to collect urine significantly impacts contamination rates:

   • Suprapubic aspiration or catheterization: Most reliable with specificity of 95-99% 1
   • Clean-catch midstream: Moderate reliability with contamination rates of 0-29% 1
   • Bag collection: Least reliable with false-positive rates of 12-83% 1

2. Presence of pyuria: According to the IDSA guidelines, both bacteriuria and pyuria should be present to diagnose a true UTI 1

   • E. faecium without accompanying pyuria may represent asymptomatic bacteriuria rather than infection

3. Colony count: Significant growth is typically defined as ≥10^5 CFU/mL from clean-catch specimens or any growth from suprapubic aspiration 1

Clinical Approach to E. faecium in Urine Culture

Asymptomatic Patients

For asymptomatic patients with E. faecium in urine (asymptomatic bacteriuria):

• Do not treat in most patient populations, including those with:

  • Indwelling catheters (strong recommendation) 1
  • Diabetes
  • Advanced age
  • Implanted urologic devices (weak recommendation) 1

• Consider treatment only before:

  • Urologic procedures with mucosal trauma (strong recommendation) 1
  • Urologic procedures requiring antimicrobial prophylaxis 1

Symptomatic Patients

For patients with symptoms of UTI and E. faecium in urine:

• Treat according to susceptibility testing
• Be aware that E. faecium has higher resistance rates than E. faecalis:
  • Only 32% of E. faecium strains may be sensitive to penicillin (vs. 96% for E. faecalis)
  • 50% sensitivity to nitrofurantoin (vs. 100% for E. faecalis) 3
  • High rates of resistance to aminoglycosides (29% with high-level resistance) 3

Important Considerations

1. Antimicrobial resistance: E. faecium has higher resistance rates than E. faecalis, including vancomycin resistance in some strains 4

2. Diagnostic challenges: Traditional culture may miss some enterococcal infections, especially in patients who have received prior antibiotics 5

3. Increasing prevalence: The prevalence of enterococcal UTIs has been steadily increasing over time, likely due to increasing use of catheterization and broad-spectrum antibiotics 6

4. Treatment options: For resistant E. faecium UTIs, potential oral agents include nitrofurantoin and fosfomycin, while parenteral options include daptomycin, linezolid, and quinupristin-dalfopristin 2

Common Pitfalls to Avoid

1. Treating all positive cultures: Not all positive cultures represent infection requiring treatment

2. Ignoring collection method: The method of urine collection significantly impacts the likelihood of contamination

3. Missing underlying conditions: E. faecium UTI may be associated with underlying urologic abnormalities (like kidney stones) that require management 5

4. Inappropriate empiric therapy: E. faecium has intrinsic resistance to many commonly used empiric antibiotics for UTI

In conclusion, E. faecium in urine culture should be interpreted in the clinical context, with consideration of collection method, presence of symptoms, pyuria, and colony count. While not typically a contaminant, treatment is not always necessary, particularly in asymptomatic patients.