Can Staphylococcus aureus in blood cultures be considered contamination, especially in vulnerable populations such as the elderly, immunocompromised individuals, or those with a history of invasive device use?

Can S. aureus be Considered Contamination in Blood Cultures?

No, Staphylococcus aureus in blood cultures should never be considered contamination and must always be treated as true bacteremia requiring immediate evaluation and treatment. 1, 2

Why S. aureus is Never a Contaminant

Unlike coagulase-negative staphylococci (which are the most frequent blood culture contaminants), S. aureus isolated from blood cultures represents true infection in >95% of cases and carries a mortality rate of 15-30%. 1, 2, 3

• Blood culture results positive for S. aureus, in the absence of any other identifiable source of infection, should immediately increase suspicion for catheter-related bloodstream infection or other serious infection. 1
• Even a single positive blood culture bottle growing S. aureus warrants full evaluation and treatment, unlike coagulase-negative staphylococci which require multiple positive cultures for diagnosis. 1

Critical Distinction from Other Skin Flora

The evidence explicitly differentiates S. aureus from true contaminants:

• Coagulase-negative staphylococci are the most frequent and more benign pathogens associated with catheter-related bloodstream infection, and diagnosis should be based on more than one set of positive blood cultures. 1
• In contrast, S. aureus infections require catheter removal and systemic antibiotic therapy for a minimum of 14 days, with 4-6 weeks treatment in cases of complications or persistent bacteremia. 1
• The skin microbe S. lugdunensis, which resembles S. aureus in virulence, should be managed as S. aureus infection, not as a contaminant. 1

Mandatory Evaluation for All S. aureus Bacteremia

Every patient with S. aureus bacteremia requires the following minimum evaluation, regardless of age or comorbidities: 1, 4, 2

• Thorough history and physical examination evaluating for source and metastatic involvement 1
• Infectious diseases consultation 1, 4
• Follow-up blood cultures at 2-4 days after initiating therapy to document clearance 1, 4, 2
• Echocardiography (at minimum transthoracic echocardiography for all patients; transesophageal echocardiography for high-risk patients) 1, 4, 2

Special Considerations in Vulnerable Populations

Elderly and Long-Term Care Facility Residents

In elderly residents of long-term care facilities, S. aureus bacteremia is more frequent and carries higher mortality, making dismissal as contamination particularly dangerous. 1, 5

• S. aureus infections are more frequent in older people and most commonly caused by colonization of intravenous catheters. 1
• The strongest predictors for mortality include pneumonia, absence of an identified infective focus, and inadequate antimicrobial therapy. 1
• Blood cultures in long-term care facilities generally have low yield for most pathogens, but when S. aureus grows, it represents true infection requiring immediate action. 1

Immunocompromised Patients

Immunocompromised patients are at highest risk for bacterial native vertebral osteomyelitis and other metastatic S. aureus infections. 1

• Patients who are elderly, immunocompromised, or active intravenous drug abusers, have indwelling central catheters, or have undergone recent instrumentation are most at risk for serious S. aureus infections. 1
• Patients with concomitant back pain and S. aureus bloodstream infection should be further investigated with imaging to rule out vertebral osteomyelitis or paraspinal abscess. 1

Patients with Invasive Devices

The presence of hemodialysis grafts/shunts, central venous access, or implantable cardiac devices significantly increases risk of S. aureus bacteremia and should never prompt consideration of contamination. 5

• Clinical characteristics associated with S. aureus bacteremia include hemodialysis graft or shunt (OR 3.22), chills (OR 2.38), and history of S. aureus infection (OR 2.68). 5
• Due to the high risk of endocarditis (25-32%), transesophageal echocardiogram is indicated unless at 72 hours after catheter removal, cultures and clinical assessments are negative. 1

High Risk of Metastatic Complications

S. aureus bacteremia causes metastatic infection in more than one-third of cases, including endocarditis (≈12%), septic arthritis (7%), vertebral osteomyelitis (≈4%), and seeding of implantable medical devices. 2, 3

• Complicated S. aureus bacteremia was present in 43% of 724 consecutive hospitalized patients in a prospective study. 6
• Prolonged S. aureus bacteremia (≥48 hours) is associated with a 90-day mortality risk of 39%. 2
• The strongest predictor of complicated S. aureus bacteremia is a positive follow-up blood culture result at 48-96 hours. 6

Common Pitfall to Avoid

Never dismiss S. aureus in blood cultures as contamination based on clinical appearance alone. Patients with S. aureus bacteremia commonly present with fever or symptoms from metastatic infection, but approximately 15% may have "afebrile" bacteremia, particularly those already receiving antimicrobial therapy. 1

• In one study, only 45% of patients with bacterial native vertebral osteomyelitis presented with fever. 1
• The absence of fever does not exclude serious S. aureus infection, especially in elderly or immunocompromised patients. 1, 7