Is Staphylococcus (Staph) epidermidis growing in a urine culture considered a pathogen or contaminant?

Staphylococcus epidermidis in Urine Culture: Contaminant vs. Pathogen

Staphylococcus epidermidis growing in a urine culture should generally be considered a contaminant rather than a true pathogen in most clinical scenarios. 1

Understanding S. epidermidis in Urine Specimens

S. epidermidis is a common skin commensal that frequently contaminates clinical specimens during collection. When evaluating its significance in urine cultures, consider:

Key Factors for Interpretation:

• Collection Method: The method used to collect the urine sample significantly impacts interpretation:

  • Bag or clean-catch specimens have high contamination rates
  • Catheterized specimens have intermediate contamination risk
  • Suprapubic aspiration has lowest contamination risk 1

• Colony Count: Lower counts (<10^5 CFU/mL) of S. epidermidis are more likely to represent contamination 1

• Presence of Pyuria: True urinary tract infections typically show both bacteriuria AND pyuria 1

• Clinical Context: Symptoms consistent with UTI increase the likelihood of pathogenicity

Clinical Decision Algorithm

1. Evaluate collection method and colony count:

   • If S. epidermidis is isolated from a bag specimen → likely contaminant
   • If isolated from catheterized urine with <10^3 CFU/mL → likely contaminant
   • If isolated from suprapubic aspiration with any growth → potential pathogen

2. Check for pyuria:

   • No pyuria + S. epidermidis → almost certainly contaminant
   • Pyuria + S. epidermidis → requires further evaluation 1

3. Consider patient factors:

   • Immunocompetent outpatient → likely contaminant
   • Indwelling urinary catheter → possible pathogen
   • Immunocompromised → possible pathogen

Special Considerations

Prosthetic Devices

S. epidermidis is a significant pathogen in patients with prosthetic devices, particularly:

• Prosthetic joint infections 1
• Indwelling vascular catheters 2
• Cerebrospinal fluid shunts 3

However, its presence in urine cultures from patients with prosthetic joints does NOT necessarily indicate prosthetic joint infection and should not automatically trigger antibiotic treatment 1.

Blood Cultures

In blood cultures, S. epidermidis is often considered a contaminant. According to HICPAC guidelines, treatment in response to a single blood culture positive for coagulase-negative staphylococcus (like S. epidermidis) should be discouraged if other blood cultures taken during the same timeframe are negative 1.

Common Pitfalls to Avoid

1. Overtreatment: Treating S. epidermidis in urine when it's a contaminant leads to unnecessary antibiotic use, potential side effects, and antimicrobial resistance 1

2. Misinterpretation of a single positive culture: A single positive culture with S. epidermidis, especially with low colony counts, should not automatically trigger treatment 1

3. Failure to repeat cultures: When in doubt about the significance of S. epidermidis, obtain a repeat culture with careful collection technique before initiating antibiotics

4. Ignoring collection method: The method of urine collection significantly impacts the likelihood of contamination 1

Conclusion

When S. epidermidis is isolated from urine, the clinical context, collection method, colony count, and presence of pyuria should guide interpretation. In most cases, especially in outpatients without indwelling devices and with proper collection techniques, S. epidermidis represents contamination rather than true infection.