Blood cultures

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.”

General indications

• In general, blood cultures are indicated for suspected or potential septicemia or sepsis to determine the causative pathogen before starting calculated antibiotic treatment.
• The most concerning signs include signs of systemic infection such as fever/hypothermia in combination with:
  • Tachycardia and tachypnea
  • Confusion
  • Severe hypotension
  • Decreased urine output
  • Elevated inflammatory markers

Indications for routine blood culture

• Conditions include:
  • Infective endocarditis
  • Sepsis or septic shock
  • Fever of unknown origin
  • Brucellosis, typhus
  • Systemic fungal infections
  • Localized infection with a systemic course, e.g.,:
    • Osteomyelitis
    • Meningitis
    • Septic arthritis
    • Pneumonia in hospitalized patients
    • Urinary tract infections (e.g., complicated pyelonephritis)
    • Abscesses
    • Catheter infections
    • Gastrointestinal infections (e.g., cholangitis)
    • Severe cellulitis
  • Potentially postsurgical infections
• High-risk groups: should have a lower threshold of suspicion
  • Newborns and infants
  • Elderly patients
  • Immunocompromised patients
  • Implants and catheter systems (e.g., heart valve prosthesis, central venous catheter)

Indications for control blood cultures

• Typically recommended within 48–72 hours of initiating antibiotic treatment
• Indicated for specific clinical syndromes, including bacteremia, infective endocarditis, intravascular catheter infection, CNS infection, unknown source of bacteremia, presence of pathogens resistant to antimicrobial agents

• 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

Preparations for blood collection

• Time of collection
  • Should be initiated before antimicrobial therapy ^[1]
  • The ideal collection time is before the onset of fever, but in practice, blood cultures are drawn when fever is detected.
  • Blood cultures should not be ordered for adult patients with leukocytosis or isolated fever without considering the pretest probability. ^[2]
• Site of collection
  • The preferred venipuncture sites are the antecubital veins or other upper extremity blood vessels.
  • Avoid collection from indwelling catheter systems.
  • No venipuncture should be performed in altered skin (e.g., inflammation, eczema).
• Apply the tourniquet and palpate the venipuncture site
• Antisepsis
  • Disinfect hands.
  • Use sterile, disposable gloves.
  • Prepare culture bottles for inoculation by removing any caps, disinfecting the site of puncture of the blood culture bottles with an appropriate disinfectant (e.g., with 2% chlorhexidine/70% isopropyl alcohol or 70% isopropyl alcohol), and allowing it to dry.
  • Disinfect the venipuncture site.
    • Use an appropriate disinfectant, e.g., 2% chlorhexidine/70% isopropyl alcohol. It is often recommended to disinfect the puncture site twice.
    • Before inserting the needle, allow the skin to dry.
    • Do not repeat venous palpation.

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.

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 ).

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.