When should sensitivities be requested for blood cultures?

When to Request Sensitivities for Blood Cultures

Blood culture sensitivities should be requested in all positive blood cultures to guide appropriate antimicrobial therapy, especially when there are signs of systemic infection including fever, chills, hypothermia, leukocytosis, left-shift of neutrophils, neutropenia, hypoalbuminemia, renal failure, or hemodynamic compromise. 1

Indications for Obtaining Blood Cultures

Blood cultures should be obtained in the following clinical scenarios:

• Fever, chills, or hypothermia in patients with suspected bloodstream infection 1
• Leukocytosis, left-shift of neutrophils, or neutropenia 1
• Hypoalbuminemia, new renal failure, or signs of hemodynamic compromise in patients with suspected infection 1
• Suspected central line-associated bloodstream infection (CLABSI) 1
• Suspected infective endocarditis 1
• Severe community-acquired pneumonia (PSI score IV or V or requiring ICU admission) 1
• Immunocompromised patients with suspected infection 1

Specific Clinical Scenarios

Community-Acquired Pneumonia

• Blood cultures are not routinely recommended for non-severe community-acquired pneumonia 1
• Blood cultures should be obtained in patients with severe pneumonia, immunocompromised state, or risk factors for unusual or resistant pathogens 1

Urinary Tract Infections

• Blood cultures are not routinely recommended for patients with urinary tract infections with systemic symptoms if a good-quality urine sample is available 1
• Blood cultures should be obtained in patients with UTI who have:
  • Antibiotic pretreatment 1
  • Chronic indwelling catheters 1
  • Immunocompromised status 1

Skin and Soft Tissue Infections

• Blood cultures are not routinely recommended for patients with cellulitis/erysipelas 1, 2
• Blood cultures should be obtained in patients with skin infections who:
  • Are immunocompromised 1
  • Have intravascular prostheses, pacemakers, or valvular prostheses 1
  • Have high risk for non-standard pathogens 1

Proper Collection Technique

To ensure optimal results and reduce contamination:

• Obtain blood via fresh venipuncture (avoid drawing through existing lines when possible) 1
• Avoid sites with skin contamination (e.g., femoral site) or compromised skin integrity 1
• Properly disinfect the skin with either 70% isopropyl alcohol or an iodine-containing solution 1
• Collect adequate blood volume (20-60 ml per culture in adults, 10-30 ml per bottle) 1, 3
• If insufficient blood is available, prioritize the aerobic bottle 1
• Obtain paired blood cultures (two sets) to increase sensitivity for detecting pathogens 4
• Collect cultures before starting antibiotics whenever possible 1

Timing of Blood Cultures

• Blood cultures should be taken as soon as possible after onset of fever or chills 1
• For patients already on antibiotics, draw cultures immediately before the next antibiotic dose when blood levels are at their lowest 1
• For suspected endocarditis, obtain 3 separate blood cultures by separate venipunctures on the first day 1

Interpretation and Antimicrobial Stewardship

• Sensitivity testing should be performed on all positive blood cultures to guide appropriate antimicrobial therapy 5
• For central line-associated infections, differential time to positivity (DTP) between catheter hub and peripheral blood samples can help determine if the catheter is the source 1
• For suspected contaminants (especially coagulase-negative staphylococci), multiple positive sets increase the likelihood of true infection versus contamination 3

Common Pitfalls to Avoid

• Collecting only one set of blood cultures (inadequate sensitivity) 3
• Insufficient blood volume (reduces sensitivity) 3, 4
• Poor aseptic technique leading to contamination 1
• Delaying collection until after antibiotics are started 1
• Overutilization in low-risk scenarios, which increases false positives 6
• Failure to collect cultures from both peripheral sites and suspected infected catheters when evaluating for catheter-related infections 1

Blood culture sensitivity testing is essential for optimizing antimicrobial therapy, reducing unnecessary antibiotic use, and improving patient outcomes by ensuring that the most appropriate and targeted therapy is administered.