When should blood cultures be obtained in patients?

When to Obtain Blood Cultures

Blood cultures should be obtained in patients with suspected infection who have an elevated temperature AND hypotension and/or tachypnea and/or delirium, OR when there is concern for antibiotic-resistant organisms that would inform the treatment regimen. 1

General Principles for Blood Culture Collection

• Obtain three to four blood cultures within the first 24 hours of fever onset, making every effort to draw cultures before initiating antimicrobial therapy 1
• For patients without indwelling vascular catheters, obtain at least two blood cultures from peripheral sites using separate venipunctures 1
• For patients with intravascular catheters, draw one blood culture by venipuncture and at least one through the catheter 1
• Draw 20-30 mL of blood per culture to maximize yield 1, 2
• Use proper skin antisepsis: 2% chlorhexidine gluconate in 70% isopropyl alcohol is preferred (30 seconds drying time), or tincture of iodine (30 seconds drying time), or povidone-iodine (2 minutes drying time) 1

Specific Clinical Scenarios Requiring Blood Cultures

Respiratory Infections

• Obtain blood cultures in patients with severe community-acquired pneumonia (PSI score IV or V or requiring ICU admission) 1
• Obtain blood cultures in patients with pneumonia who have risk factors for resistant pathogens 1
• Obtain blood cultures in immunocompromised patients with pneumonia 1

Urinary Tract Infections

• Blood cultures are not routinely needed in patients with UTI with systemic symptoms without anatomical abnormalities if a good-quality urine sample for culture is available 1
• Obtain blood cultures in patients with UTI with systemic symptoms who:
  • Have received antibiotic pretreatment 1
  • Have a chronic indwelling catheter 1
  • Are immunocompromised 1

Skin and Soft Tissue Infections

• Blood cultures are not routinely needed in patients with cellulitis/erysipelas 1
• Obtain blood cultures in patients with cellulitis/erysipelas who:
  • Are immunocompromised 1
  • Have high risk of non-standard pathogens 1
  • Have intravascular prosthesis, pacemaker, or valvular prosthesis 1

Intra-abdominal Infections

• Obtain blood cultures in patients with suspected intra-abdominal infections who have fever AND hypotension/tachypnea/delirium 1
• Obtain blood cultures when there is concern for antibiotic-resistant organisms in intra-abdominal infections 1

Suspected Endocarditis

• Blood cultures are essential for patients with fever of unexplained origin and a pathological heart murmur, history of heart disease, or previous endocarditis 1
• For suspected endovascular infections, draw blood cultures by timed intervals to demonstrate continuous bacteremia 1

Predictors of True Bacteremia

• Major criteria that warrant blood cultures include: temperature >39.5°C, indwelling vascular catheter, or clinical suspicion of endocarditis 3
• Minor criteria include: temperature 38.3-39.4°C, age >65 years, chills, vomiting, hypotension, neutrophilia >80%, WBC >18k, bands >5%, platelets <150k, and creatinine >2.0 3
• Presence of SIRS and elevated CRP are powerful predictors of positive blood cultures 4

Common Pitfalls to Avoid

• Delaying antimicrobial therapy more than 45 minutes to obtain blood cultures in critically ill patients with suspected sepsis 5
• Drawing only a single blood culture, which significantly reduces sensitivity and makes contamination difficult to interpret 1, 6
• Obtaining blood cultures from newly inserted peripheral venous catheters, which leads to high contamination rates 1
• Routinely obtaining blood cultures in conditions with low yield such as cellulitis, simple pyelonephritis, and community-acquired pneumonia in otherwise healthy patients 7

Follow-up Blood Cultures

• Additional blood cultures should only be drawn when there is clinical suspicion of continuing or recurrent bacteremia/fungemia, or for test of cure 48-96 hours after initiating appropriate therapy 1
• Additional cultures should always be paired, not single specimens 1