In a clinically well patient with one blood culture bottle growing Staphylococcus epidermidis and the other bottle negative after 73 hours, and a positive nasal MRSA screen, should I continue cefazolin (Ancef) and repeat blood cultures?

Single Bottle Staph Epidermidis: Likely Contamination - Discontinue Ancef and Repeat Cultures

In a clinically well patient with only one of two blood culture bottles growing Staphylococcus epidermidis after 73 hours, this almost certainly represents contamination rather than true bacteremia, and you should discontinue cefazolin (Ancef). However, you should repeat blood cultures to confirm clearance, particularly given the positive nasal MRSA screen.

Key Clinical Reasoning

Why This is Contamination

The single positive bottle with S. epidermidis in a clinically well patient is a classic presentation of contamination. True coagulase-negative staphylococcal (CoNS) bacteremia typically requires:

• Multiple positive blood culture bottles
• Clinical signs of sepsis
• Presence of indwelling devices (central lines, prosthetic valves, etc.)

Research demonstrates that low bacterial density with prolonged time to positivity (>20 hours) strongly suggests contamination, with favorable outcomes even without antibiotic therapy 1. In one study, 71% of patients with low colony counts (<10 CFU/ml) and prolonged time to positivity received no antibiotics and 83% retained their central venous catheters, yet all had favorable outcomes 1.

The MRSA Colonization Issue

Your patient's positive nasal MRSA screen is a red herring in this context. Nasal colonization does not equal active infection. The MRSA screen is relevant for:

• Contact precautions
• Surgical prophylaxis decisions
• Future risk stratification

But it does NOT justify continuing antibiotics for what appears to be a contaminated blood culture.

Specific Action Plan

1. Discontinue Cefazolin

• Stop the Ancef immediately - there is no indication to treat contamination
• The patient is clinically well, which is your most important clinical data point
• Continuing unnecessary antibiotics increases risk of:
  • C. difficile infection
  • Drug resistance
  • Adverse drug reactions
  • Unnecessary healthcare costs

2. Repeat Blood Cultures - Yes, Absolutely

You must repeat blood cultures to document that there is no true bacteremia 2. The 2023 guidelines on staphylococcal bacteremia emphasize that repeat blood cultures are part of the minimum evaluation for any positive blood culture 2.

Draw at least two sets of blood cultures (aerobic and anaerobic bottles from separate venipunctures) because:

• If these are negative, it confirms your suspicion of contamination
• If S. epidermidis grows from multiple bottles, you'll need to reconsider true CoNS bacteremia
• If S. aureus grows (given the MRSA colonization), you'll need to initiate appropriate therapy

3. Clinical Monitoring

Watch for any signs that would suggest true infection:

• Fever or hemodynamic instability
• New heart murmur (endocarditis concern)
• Signs of line infection if central access present
• Any prosthetic devices that could be seeded

Common Pitfalls to Avoid

Pitfall #1: Overreacting to MRSA colonization

• Nasal MRSA colonization occurs in 1-2% of the general population
• It does not require treatment unless there is active infection
• Do not conflate colonization with the contaminated blood culture

Pitfall #2: Treating contamination "just to be safe"

• This is not safe - it's harmful
• Unnecessary antibiotics cause real patient harm
• The clinical picture (well-appearing patient, single bottle, CoNS organism) overwhelmingly suggests contamination

Pitfall #3: Not repeating cultures

• Even though you're confident this is contamination, you need documentation
• The repeat cultures protect you medicolegally and clinically
• They ensure you're not missing an early or low-grade bacteremia

If Repeat Cultures Are Positive

Should the repeat cultures grow S. epidermidis from multiple bottles, then you would need to:

• Reassess for prosthetic devices or indwelling catheters
• Consider echocardiography if any cardiac risk factors
• Initiate appropriate therapy based on susceptibilities
• Consider vancomycin (not cefazolin) given the MRSA colonization suggests this patient may harbor resistant organisms

Bottom Line

Stop the Ancef, repeat the blood cultures, and observe the patient clinically. The constellation of findings - single positive bottle, S. epidermidis (a common skin contaminant), prolonged time to positivity (73 hours), and clinically well patient - makes contamination the overwhelmingly likely diagnosis. Continuing antibiotics in this scenario provides no benefit and causes potential harm.