Is a blood culture growing Staphylococcus hominis (often misidentified as Staphylococcus hominid) likely a contaminant?

Is Staphylococcus hominis in Blood Culture a Contaminant?

Staphylococcus hominis isolated from a single blood culture is most likely a contaminant and should not prompt empiric antibiotic therapy unless multiple blood cultures are positive or there are specific high-risk clinical features present. 1, 2

Key Diagnostic Criteria to Distinguish True Infection from Contamination

Multiple Positive Cultures Are Essential

• At least 2 blood cultures positive for coagulase-negative staphylococci (CoNS) including S. hominis within 48-72 hours is the minimum threshold to consider true bacteremia, according to the Infectious Diseases Society of America 1
• If only 1 of 2 blood cultures drawn simultaneously is positive for CoNS, this is highly likely contamination and vancomycin should be withheld 1
• Approximately 65-94% of CoNS-positive blood cultures represent contamination rather than true bloodstream infection 2
• In one study, 85% of all coagulase-negative staphylococci isolated were judged to be contaminants 3

Time to Positivity (TTP) Helps Differentiate

• Growth of S. hominis in <48 hours is significantly associated with true bacteremia 4, 3
• For true S. hominis bacteremia, most time to positivity is <48 hours, with catheter-related bloodstream infections typically showing TTP <24 hours 4
• Delayed time to positivity (>48 hours) further supports contamination 5

Positive Bottle Detection Pattern

• All four bottles in two sets of blood cultures being positive strongly indicates true bacteremia - this occurred in 14/14 (100%) of confirmed S. hominis bacteremia cases 4
• Growth in both aerobic and anaerobic bottles in 85% of blood culture sets is associated with true bacteremia, compared to only 30% in contaminated cultures 3
• Single positive blood culture with negative concurrent cultures likely represents contamination 2

Clinical Risk Factors That Increase Likelihood of True Infection

High-Risk Scenarios for True S. hominis Bacteremia

• Presence of indwelling central venous catheter or other prosthetic devices - seen in 87.9% of true CoNS bacteremia cases 1, 3
• Clinical evidence of catheter-related infection such as exit site infection or tunnel infection 1
• Recent healthcare exposure, hemodialysis, chronic wounds, or long-term care facility residence 1
• Immunocompromised patients with indwelling or implanted foreign bodies 6

Clinical Signs Supporting True Infection

• Persistent bacteremia despite appropriate initial management 1
• Presence of fever, hypotension, or other signs of sepsis 5
• Clinical correlation that supports infection rather than contamination 7

Recommended Clinical Approach

Initial Assessment Algorithm

1. Determine number of positive blood cultures: Single positive = likely contaminant; ≥2 positive = consider true infection 7, 1
2. Evaluate time to positivity: <48 hours supports infection; >48 hours supports contamination 4, 3
3. Assess bottle pattern: All bottles positive strongly suggests infection; single bottle positive suggests contamination 4
4. Review clinical context: Presence of indwelling devices, immunosuppression, signs of sepsis 1, 5

Management Based on Assessment

• If likely contamination (single positive culture, no risk factors): Avoid unnecessary antibiotic therapy, particularly vancomycin; obtain additional blood cultures to confirm absence of bacteremia 2
• If true infection suspected (multiple positive cultures, risk factors present): Initiate vancomycin therapy at 40 mg/kg/day IV divided every 8-12 hours (maximum 2 g daily) 1
• If uncertain: Obtain repeat blood cultures before initiating antimicrobial therapy; for patients with indwelling catheters, obtain paired blood cultures from the catheter and a peripheral site 5

Common Pitfalls to Avoid

Critical Errors in Management

• Don't automatically treat a single positive CoNS blood culture with vancomycin - this promotes unnecessary antibiotic use and resistance 1, 2
• Don't ignore proper blood culture collection technique - meticulous skin antisepsis with chlorhexidine or 2% iodine tincture reduces contamination rates 7, 5
• Don't overlook that 58-87% of CoNS isolates are methicillin-resistant, so if treatment is warranted, vancomycin is appropriate empirical therapy 1

Special Considerations

• Drawing three to four blood cultures with appropriate volume (20-30 mL each) from separate sites within the first 24 hours is the most effective way to discern true pathogen from contaminant 7
• Catheter-drawn blood cultures have a higher risk of contamination (false positives) compared to peripheral venipuncture 7
• Laboratories should abbreviate workup of common blood culture contaminants including coagulase-negative staphylococci, with a comment instructing clinicians to contact the laboratory if clinically significant 7

Treatment Duration If True Infection Confirmed

• Uncomplicated CoNS bacteremia requires 10-14 days of antimicrobial therapy if prompt clinical response and no complications 1
• Complicated infection (septic thrombosis, endocarditis, osteomyelitis) requires 4-8 weeks of therapy depending on the site 1
• Remove or replace central venous catheters if multiple blood cultures are positive for CoNS 1