When should I consider getting blood cultures?

When to Obtain Blood Cultures

Blood cultures should be obtained in patients with suspected bloodstream infection, particularly when there are signs of systemic infection including fever, chills, hypothermia, leukocytosis, left-shift of neutrophils, neutropenia, hypoalbuminemia, renal failure, or hemodynamic compromise. 1, 2

Primary Indications for Blood Cultures

• Obtain blood cultures in patients with suspected sepsis who have elevated temperature AND hypotension and/or tachypnea and/or delirium 1
• Blood cultures are indicated when there is concern for antibiotic-resistant organisms that would inform treatment regimen 1
• Obtain cultures in patients with fever, chills, or hypothermia as these are specific indications of potential bacteremia 1, 2
• Blood cultures should be taken in patients with signs of hemodynamic compromise, including hypotension not responsive to initial fluid resuscitation 1
• Consider blood cultures in patients with leukocytosis, left-shift of neutrophils, or neutropenia 1, 2
• Blood cultures are recommended for patients with suspected infective endocarditis 2

Clinical Decision Rule for Blood Cultures

The Shapiro decision rule can help determine when blood cultures are indicated 3:

Major Criteria:

• Temperature > 39.5°C (103.0°F) 3
• Indwelling vascular catheter 3
• Clinical suspicion of endocarditis 3

Minor Criteria:

• Temperature 38.3-39.4°C (101-102.9°F) 3
• Age > 65 years 3
• Chills 3
• Vomiting 3
• Hypotension (systolic blood pressure < 90 mm Hg) 3
• Neutrophil percentage > 80% 3
• White blood cell count > 18,000 3
• Bands > 5% 3
• Platelets < 150,000 3
• Creatinine > 2.0 3

Blood cultures are indicated if at least one major criterion or two minor criteria are present. 3

Specific Clinical Scenarios

• Obtain blood cultures in patients with severe community-acquired pneumonia (requiring ICU admission) 2
• Blood cultures are recommended in immunocompromised patients with suspected infection 2
• Obtain blood cultures in patients with complicated pyelonephritis 2, 4
• Blood cultures are not recommended for patients with cellulitis, simple pyelonephritis, and non-severe community-acquired pneumonia 2, 5

Proper Collection Technique

• Obtain blood via fresh venipuncture rather than through existing lines when possible 1, 2
• Avoid sites with skin contamination (e.g., femoral site) or compromised skin integrity 1, 2
• Disinfect the skin with either 70% isopropyl alcohol or an iodine-containing solution 1, 2
• Collect adequate blood volume (20-60 ml per culture in adults, 10-30 ml per bottle) 1, 2
• Obtain at least two sets of blood cultures (both aerobic and anaerobic bottles) 1, 6
• If insufficient blood is available, prioritize the aerobic bottle 1

Timing of Blood Cultures

• Collect blood cultures before starting antibiotic therapy whenever possible, as antibiotics significantly reduce pathogen detection (50.6% positivity before antibiotics vs. 27.7% during antibiotics) 1, 7
• If the patient is already on antibiotics, draw cultures immediately before the next antibiotic dose when blood levels are lowest 1, 2
• Take blood cultures as soon as possible after onset of fever or chills 1, 2
• For suspected endocarditis, obtain 3 separate blood cultures by separate venipunctures on the first day 2

Common Pitfalls to Avoid

• Avoid poor aseptic technique leading to contamination, which can result in unnecessary treatments and increased costs 1, 2
• Don't delay obtaining blood cultures when sepsis is suspected - they should be collected within 45 minutes of deciding to start antimicrobials 1
• Avoid relying solely on isolated fever or leukocytosis without considering the pretest probability of bacteremia 8
• Don't collect only one set of blood cultures, as this is rarely sufficient to establish or rule out bacteremia 6
• Avoid drawing cultures through existing lines unless evaluating for catheter-related infections 2