Most positive blood cultures represent a true infection, however contamination should be considered in cases of atypical organisms isolated in only a single blood culture bottle in a clinically well patient with no other evidence of infection.

Consider following factors in assessing clinical significance of positive blood culture [1]

• Clinical likelihood of bacteraemia
  • Indication for taking blood culture initially
    • {{ Each indication has varying likelihood of bacteraemia [1]
      • High risk (>50%)
        • Discitis, vertebral osteomyelitis or epidural abscess
        • Septic shock
        • Native septic joint 
        • Catheter related bloodstream infections 
      • Moderate risk (20-50%)
        • Pyelonephritis
        • Cholangitis 
        • Severe sepsis
        • Shaking chills in febrile patients
      • Low risk (<20%)
        • Cellulitis 
        • Community acquired pneumonia
        • Lower urinary tract infection
        • Isolated fever 
        • Fever within first 48hrs after surgery }}
  • Evidence of haemodynamic instability
  • Elevated CRP or WCC
• Organism isolated {{call out box to table 1 – see below}} 
• Number of positive blood cultures
  • If both anaerobic and aerobic bottles positive OR multiple cultures (taken from different sites) are positive with same organism, more likely to be clinically significant [2]
• Time to positivity
  • If becomes positive >72hrs after collection, consider contamination 

{{Table 1