Yes, you absolutely need to repeat blood cultures when one bottle is positive for Staphylococcus aureus
Repeat blood cultures are a mandatory component of the minimum evaluation for any S. aureus bacteremia, regardless of whether your initial set shows one positive and one negative bottle. 1
Why This Matters for Patient Outcomes
The 2023 Clinical Infectious Diseases guidelines explicitly list "repeat blood cultures" as part of the minimum evaluation for all S. aureus bacteremia cases, alongside thorough history/exam and transthoracic echocardiography 1. This is not optional—it's fundamental to risk stratification and treatment decisions.
Key Clinical Reasoning:
Even a single positive S. aureus culture represents true bacteremia in 85-90% of cases 2, 3. Your scenario of one positive and one negative bottle does not rule out significant infection:
- 15% of patients with a single positive S. aureus bottle will have positive follow-up cultures 4
- This rate jumps to 19% for MRSA versus 9% for MSSA 4
- Importantly, 50% of patients with persistent bacteremia on follow-up cultures are afebrile, so clinical appearance is unreliable 4
The "Skip Phenomenon" Problem
A critical pitfall: 13% of S. aureus bacteremia cases demonstrate intermittent negative blood cultures before complete clearance 5. This means:
- A single negative culture has only 87-93% predictive value for resolution of bacteremia on days 1-3 5
- 4% of patients show the skip phenomenon—negative cultures sandwiched between positive ones 6
- Patients with the skip phenomenon had significantly longer bacteremia duration (median 10 vs 8 days) 6
The clinical consequence: Patients who don't get follow-up cultures have a 17% relapse rate versus 2% in those who do get them (p=0.008) 4.
Practical Implementation
Timing: Obtain follow-up blood cultures at 2-4 days after the initial positive culture and continue as needed until clearance is documented 1.
What constitutes clearance: Do not accept a single negative blood culture as proof of clearance. The evidence strongly suggests obtaining serial negative cultures, particularly if considering the patient for "low-risk" SAB classification or planning device reimplantation 5, 6.
Risk stratification depends on this: Your patient cannot be classified as "low-risk SAB" (which requires blood cultures clearing in <48 hours) without actually documenting clearance 1. Misclassification could lead to inadequate workup for metastatic foci and inappropriate treatment duration.
The Bottom Line
Your initial finding of one positive and one negative bottle for S. aureus does not change standard management. Treat this as S. aureus bacteremia, obtain follow-up cultures at 2-4 days, and continue surveillance cultures until you document clearance with serial negative results. The mortality implications of persistent bacteremia (attributable mortality increases with each day of culture positivity) make this non-negotiable 1.