How to Rule Out Suspected Bloodstream Infection
Obtain at least two sets of blood cultures (one drawn peripherally by venipuncture and one from any vascular catheter if present) as soon as possible after fever onset or when clinical indicators suggest infection, before starting antibiotics whenever feasible. 1, 2, 3
When to Obtain Blood Cultures
Blood cultures are indicated when any of the following are present 2:
- Fever, chills, or hypothermia
- Leukocytosis, left-shift of neutrophils, or neutropenia
- Hemodynamic compromise (hypotension, signs of organ dysfunction)
- Hypoalbuminemia with suspected infection
- Development of acute renal failure in the setting of suspected infection
Timing Considerations
- Draw blood cultures immediately after fever onset or chills develop (bacteria are rapidly cleared from blood, and fever typically follows bacteremia by 30-90 minutes) 2
- If the patient is already on antibiotics, draw cultures immediately before the next scheduled antibiotic dose when drug levels are lowest 2
- Use blood culture media containing antibiotic-adsorbing substances (BacT/Alert FAN or BACTEC Plus/F) for patients already receiving antimicrobials 2
Proper Collection Technique
Blood Volume and Number of Sets
- Adults: 20-30 mL total blood per culture set (distributed across 2+ bottles depending on manufacturer) 3
- Minimum of 2 separate blood culture sets from different venipuncture sites 1, 4, 3
- At least one set must be drawn peripherally by fresh venipuncture 1, 2
- If catheter-related infection is suspected, draw one set through the catheter and one peripherally 1, 4
Site Selection and Skin Preparation
- Avoid contamination-prone sites (femoral vein, areas with burns or dermatological disease) 2
- Swab skin twice with 70% isopropyl alcohol or iodine-containing solution 2
- Any purulence at catheter exit sites should be Gram stained and cultured separately 1, 4
Catheter-Specific Evaluation
When intravascular catheter infection is suspected 1, 4:
Short-term Catheters (peripheral IV, non-tunneled central lines, arterial catheters)
- Remove and culture the catheter if there is:
- Purulence or inflammation at insertion site
- Septic shock or embolic phenomena
- Persistent fever without other source
- Culture a 5-7 cm intracutaneous segment using semiquantitative (roll-plate) or quantitative methods 4
- Growth of ≥15 CFU by roll-plate or ≥10² CFU by sonication indicates catheter colonization 4
Long-term Catheters (tunneled CVCs, implanted ports)
- May use paired quantitative blood cultures (catheter vs. peripheral) with a ratio >3:1 suggesting catheter source 4
- Differential time to positivity (catheter culture positive ≥2 hours before peripheral culture) also indicates catheter-related infection 4
Adjunctive Laboratory Markers
While not diagnostic alone, these support clinical decision-making 2:
- Procalcitonin (PCT): Level <0.5 ng/mL for 3 consecutive days effectively rules out bloodstream infection (sensitivity 98.1%, specificity 79.5%) 5
- C-reactive protein (CRP): Level ≥50 mg/L has 98.5% sensitivity and 75% specificity for sepsis 2
- Sequential measurements are more valuable than single values 2
Critical Pitfalls to Avoid
- Single blood cultures are not recommended (except neonates) - they cannot distinguish contamination from true infection 1
- Do not delay cultures for antibiotic administration in hemodynamically unstable patients - draw cultures first, then start empiric therapy immediately 1
- Do not routinely remove catheters in mildly ill patients with fever - obtain cultures first and assess clinically 4, 6
- Do not use qualitative broth cultures of catheter tips - only semiquantitative or quantitative methods are recommended 4
Special Populations
Suspected Fungemia
- Obtain fungal blood cultures in patients with epidemiologic or host risk factors (immunosuppression, prolonged antibiotics, TPN) 7
- Standard blood culture broths detect most Candida species, but lysis-centrifugation may be needed for filamentous fungi 3