Blood cultures

Last updated: March 16, 2022

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Blood cultures allow the identification of pathogens, such as bacteria and fungi, in blood and their specific resistance testing. Knowing the pathogen enables effective, individualized antibiotic treatment, which reduces mortality, improves prognosis, minimizes the length of hospital stays, and reduces the growing number of antibiotic-resistant pathogens. Several studies have shown that successful pathogen detection is much less frequent in a clinical setting than under controlled study conditions, which is mainly attributed to errors in blood culture sampling. Therefore, the use of the correct technique is vital for blood culture collection.

See also “Bacteremia.”

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General indications

Indications for routine blood culture

Indications for control blood cultures

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  • Tourniquet
  • Disinfectant (e.g., chlorhexidine, iodopovidone)
  • Sterile swab
  • Sterile gloves
  • Blood culture sets: when possible, a set of aerobic and anaerobic blood culture bottles should be collected
  • Collection needles
  • Biomedical waste container
  • Bandage

Procedure/applicationtoggle arrow icon

Preparations for blood collection

Sterile blood collection is essential. Insufficient disinfection leads to an increased risk of blood culture contamination and false positives.

Blood culture collection

  • Blood collection and inoculation of the blood culture sets
    • Use two blood culture bottles, one for aerobes and one for anaerobes. [3]
    • First fill the aerobic bottle, then the anaerobic bottle.
    • For adults, collect 10–20 cc and 1–3 cc for a child for each blood culture set [3]
  • Labeling and transport
    • Label blood culture bottles.
    • Fill in the requisition forms and arrange rapid transport
      • Ensure patient data is correct (e.g., surname, date of birth)
      • Date, time, site of collection
      • Enter relevant medical history (e.g., underlying conditions, risk factors, previous antibiotic treatment, pre-existing conditions, suspected diagnosis)
      • Storage and transport at room temperature

Collection of paired blood culture sets

  • Performed when there is a suspicion of catheter-related infections
  • One blood culture set is taken from the peripheral vein and one set from the possibly infected catheter.
  • Request differential time to positivity (DTP)
    • Compares the length of time required until there is positive pathogen detection between the blood culture set from the peripheral vein and the suspected catheter
    • If the blood culture set from the catheter is positive more than 2 hours before the peripheral venous set, then a catheter-associated infection is highly likely.
    • If both blood culture sets are positive at a similar point in time, the current infection is most likely not due to the catheter.

Waiting for a rise in fever is not recommended for blood culture collection. Blood collection should be rapidly performed in the presence of indications and prior to initiating treatment.

Interpretation/findingstoggle arrow icon

Positive blood culture

  • Interpretation will depend on the organisms identified.
  • Assess whether the pathogen identified is a plausible cause of the presenting infection, due to possible contamination, or if the significance of these findings is limited (e.g., only one blood culture set was collected ).
Positive blood culture interpretation [4]
Bacteremia is likely (true positive) Contamination should be considered (false positive)
Further steps
  • Repeat blood culture sampling and analysis.

Although coagulase-negative staphylococci found in blood cultures are most often skin flora contaminants, they may indicate true bacteremia in patients with clinical evidence of a bacterial infection (e.g., cellulitis), immune deficiency, and/or intravascular medical devices (e.g., central venous line, pacemakers). [4]

Negative blood culture

Before ruling out bacteremia, consider the following steps:

  • Determine whether it is likely the result could be a false-negative. Consider if:
    • Collection performed during current antibiotic treatment
    • Insufficient inoculation volume of each blood culture
    • Prolonged transportation time
  • Assess whether the pathogen identified is a plausible cause of the presenting infection.
  • If necessary, arrange for new blood culture sampling and analysis.

Referencestoggle arrow icon

  1. Cheng MP, Stenstrom R, Paquette K, et al. Blood Culture Results Before and After Antimicrobial Administration in Patients With Severe Manifestations of Sepsis: A Diagnostic Study.. Ann Intern Med. 2019; 171 (8): p.547-554.doi: 10.7326/M19-1696 . | Open in Read by QxMD
  2. Coburn B, Morris AM, Tomlinson G, Detsky AS. Does this adult patient with suspected bacteremia require blood cultures?. JAMA. 2012; 308 (5): p.502-11.doi: 10.1001/jama.2012.8262 . | Open in Read by QxMD
  3. Collecting Cultures: a Clinician Guide. Updated: August 1, 2009. Accessed: May 3, 2021.
  4. Doern GV, Carroll KC, Diekema DJ, et al. Practical Guidance for Clinical Microbiology Laboratories: A Comprehensive Update on the Problem of Blood Culture Contamination and a Discussion of Methods for Addressing the Problem. Clin Microbiol Rev. 2019; 33 (1).doi: 10.1128/cmr.00009-19 . | Open in Read by QxMD
  5. Hahn J-M. Checkliste Innere Medizin. Thieme Verlag (2010) ; 2010

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