MDM Templates

True Bacteremia (Staph aureus)

Patient presents for evaluation of positive blood cultures growing ***. They are not septic appearing, are tolerating PO, are ambulatory, and are maintaining appropriate oxygen saturations. No obvious source of infection is identified on exam.

Despite clinical stability, Staphylococcus aureus bacteremia carries significant risk for endocarditis, metastatic infection, and mortality even in well-appearing patients. History and exam do not reliably exclude endovascular seeding.

Plan: Repeat blood cultures prior to antibiotics. Initiate vancomycin IV pending sensitivities.
Disposition: Admit for IV antibiotics, serial blood culture clearance documentation, and echocardiogram. Infectious disease consultation for duration of therapy guidance.

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Likely Contaminant

Patient presents for evaluation of positive blood cultures growing ***. They are not septic appearing, tolerating PO, ambulatory, and maintaining appropriate oxygen saturations. Clinical presentation is not consistent with serious bacterial illness.

Growth of coagulase-negative Staphylococcus / Corynebacterium / Propionibacterium / Bacillus species in a single bottle from an otherwise well patient is most consistent with blood culture contaminant. History and exam lower suspicion for true bacteremia, endocarditis, or occult deep-space infection.

Plan: Repeat blood cultures drawn from two separate sites for outpatient follow-up.
Disposition: Discharge with return precautions for fever, rigors, or worsening symptoms. Follow up with PCP within 24 hours for culture results. Patient understands that if repeat cultures are positive, they may require admission for IV antibiotics.

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Clinical Education

True Pathogen vs Contaminant

The single most important question: is this organism ever a contaminant? S. aureus, E. coli, Klebsiella, Pseudomonas, and Streptococcus species are virtually always true pathogens when isolated from blood cultures. These get admitted and treated.^[1]

Coagulase-negative Staphylococci (CoNS) are the most common blood culture contaminant, representing 70-80% of false positives. The key distinction: a single positive bottle in an otherwise well patient is likely contamination. Two or more bottles growing the same organism with identical sensitivities suggests true infection.^[1]

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Staph Aureus Bacteremia

S. aureus bacteremia is never a contaminant and is never minor. Even in a well-appearing patient, mortality ranges from 20-40% without appropriate treatment. Mandatory workup includes repeat cultures to document clearance, echocardiogram (TEE preferred over TTE for sensitivity), and ID consultation. Minimum 14 days IV antibiotics for uncomplicated bacteremia; 4-6 weeks if endocarditis or metastatic infection identified.^[2]

ID consultation reduces mortality. A meta-analysis showed that mandatory ID consultation for S. aureus bacteremia reduces mortality by approximately 50%. This is one of the strongest interventions in infectious disease.^[3]

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Common Contaminant Organisms

Organism	Contaminant Rate	When It’s Real
Coagulase-negative Staph (S. epidermidis, S. hominis, S. capitis)	~85% contaminant	Prosthetic valve, indwelling catheter, immunosuppressed; 2+ bottles same organism/sensitivities
Corynebacterium species	~90% contaminant	Prosthetic devices, immunocompromised
Propionibacterium (Cutibacterium) acnes	~95% contaminant	Prosthetic joint infection, neurosurgical hardware
Bacillus species (not anthracis)	~95% contaminant	IV drug use, indwelling catheters

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Disposition Guidance

Admit: S. aureus (always), any organism in a toxic-appearing patient, any organism in an immunocompromised patient, 2+ bottles growing the same organism, known endovascular hardware with positive cultures.^[2]

Discharge with close follow-up: Single bottle growing likely contaminant in a well-appearing, immunocompetent patient without endovascular hardware. Repeat cultures must be drawn before discharge. PCP follow-up within 24 hours is mandatory.^[1]

Never discharge without a plan for culture follow-up. The worst outcome is a patient discharged with “contaminant” cultures who actually has endocarditis.

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References

1. Hall KK, Lyman JA. Updated Review of Blood Culture Contamination. Clin Microbiol Rev. 2006;19(4):788-802. PubMed
2. Holland TL et al. Clinical Management of Staphylococcus aureus Bacteremia: A Review. JAMA. 2014;312(13):1330-1341. PubMed
3. Vogel M et al. Infectious Disease Consultation for Staphylococcus aureus Bacteremia: A Systematic Review and Meta-Analysis. J Infect. 2016;72(1):19-28. PubMed