From the Guidelines
Blood cultures are most effective when collected before the administration of intravenous antibiotics, as the effectiveness of blood cultures decreases significantly after antibiotic administration, with detection rates dropping by approximately 35-40% after just one antibiotic dose. The reduced effectiveness occurs because antibiotics begin killing bacteria in the bloodstream shortly after administration, lowering the bacterial concentration below detectable levels, which can lead to false-negative results while the patient still has an active infection 1.
Key Considerations
- In critically ill patients where infection is strongly suspected, delaying antibiotics to obtain cultures is not recommended as this could worsen outcomes, and instead, cultures should be drawn immediately before starting antimicrobial therapy, even if this means a brief delay of minutes in antibiotic administration 1.
- If blood cultures must be obtained after antibiotics have been started, they should be collected as soon as possible, ideally within the first few hours after the initial dose, and multiple sets (typically 2-3 sets from different venipuncture sites) should be collected to maximize yield 1.
- Specialized blood culture bottles containing antibiotic-binding resins or charcoal (such as BACTEC or BacT/Alert systems) should be used when available to improve detection rates in patients who have received antibiotics 1.
Clinical Recommendations
- Obtain blood cultures before initiating antimicrobial therapy if possible, with a goal of no substantial delay in the start of antimicrobials, and consider a delay of up to 45 minutes as acceptable if it allows for the collection of cultures 1.
- Use strict aseptic technique for blood culture collection, and consider using 2% chlorhexidine gluconate in 70% isopropyl alcohol for skin disinfection, allowing for 30 seconds of drying time before proceeding with the culture procedure 1.
- Label blood cultures with the exact time, date, and anatomic site from which they were taken, and draw 20-30 mL of blood per culture to optimize yield 1.
From the Research
Effectiveness of Blood Cultures After IV Antibiotics
- The effectiveness of blood cultures after administration of Intravenous (IV) antibiotics is a topic of interest in the medical field, with several studies investigating this issue 2, 3, 4, 5, 6.
- A study published in 2019 found that the blood culture positivity rate decreases after administration of IV antibiotics, with a significant reduction in positivity rate observed within the first hour of antibiotic administration 2.
- Another study published in 2019 found that the sensitivity of blood cultures obtained after initiation of antimicrobial therapy was 52.9%, indicating that a significant proportion of blood cultures may still be positive after antibiotic administration 3.
- The timing of blood culture collection is crucial, with one study finding that increasing time between antibiotic administration and post-antibiotic culture was an independent predictor for blood culture sterilization 6.
- The volume of blood culture is also an important factor, with a study finding that blood culture volume was an independent predictor for blood culture positivity 6.
- Clinical decision support systems can improve blood culture collection before IV antibiotic administration in the emergency department, leading to increased timely blood culture collections and improved outcomes 5.
Key Findings
- Blood culture positivity rate decreases after administration of IV antibiotics 2, 3.
- Sensitivity of blood cultures obtained after initiation of antimicrobial therapy is around 52.9% 3.
- Timing of blood culture collection is crucial, with increasing time between antibiotic administration and post-antibiotic culture leading to blood culture sterilization 6.
- Blood culture volume is an important factor, with larger volumes leading to increased positivity rates 6.
- Clinical decision support systems can improve blood culture collection before IV antibiotic administration 5.