🔬 Blood Cultures

Blood cultures are an essential microbiological investigation used to detect the presence of bacteria or fungi in the blood (bacteraemia or fungaemia). The process involves inoculating aerobic and anaerobic bottles with venous blood using a strict aseptic technique. These cultures are monitored for several days, and any growth is identified and tested for antibiotic sensitivity.

Blood cultures should be taken in any patient with signs of systemic infection or sepsis (fever >38°C, rigors, tachycardia, hypotension, or unexplained confusion). They are mandatory before starting antibiotics in suspected sepsis, meningitis, endocarditis, or discitis. They may also be indicated in patients with unexplained pyrexia or those who are immunocompromised with a low-grade fever.

Aseptic technique is critical. Using a 'blood culture pack', skin should be cleaned with 2% chlorhexidine in 70% isopropyl alcohol. Two bottles (aerobic and anaerobic) constitute one 'set'. Ideally, two sets should be taken from different sites. For suspected endocarditis, three sets should be taken over a 24-hour period. Bottles should be filled with 8-10mL of blood each to optimize yield.

A normal result is reported as 'No growth after 5 days' (though labs may monitor for longer in suspected endocarditis or fungal infections). This suggests that there was no detectable viable bacteria or fungi in the blood sample at the time of collection, although it does not absolute rule out deep-seated infection.

Interpretation requires distinguishing 'pathogens' from 'contaminants'. Growth of S. aureus is almost always significant. Growth of skin flora (e.g., Cutibacterium or Coag-neg Staph) in only one of several bottles often suggests contamination during collection. Time to positivity (TTP) is also useful; very rapid growth usually indicates a high bacterial load. All results must be discussed with microbiology.

Growth of a pathogenic organism (e.g., S. aureus, E. coli, S. pneumoniae) in one or more bottles constitutes a positive result. If the same organism grows in multiple sets, it strongly suggests true bacteraemia. Significant findings include skin contaminants (e.g., Coagulase-negative Staphylococci) growing in multiple bottles or from a sterile site (e.g., a central line), which may indicate a line infection or endocarditis.

Blood cultures are the definitive method for identifying the causative pathogen in septicaemia and endocarditis. They allow for antimicrobial susceptibility testing, which is crucial for 'stepping down' from broad-spectrum to narrow-spectrum (targeted) therapy. This helps improve patient outcomes and supports antimicrobial stewardship by reducing the use of inappropriate antibiotics.

Taking blood from an existing intravenous cannula is a major pitfall as it significantly increases the risk of contamination; fresh peripheral stabs are preferred. Another pitfall is under-filling the bottles, which significantly reduces the sensitivity of the test. Failing to take cultures before starting antibiotics is a critical clinical error in the management of sepsis.

Sensitivity is reduced if the patient has already received antibiotics. Blood cultures will not identify fastidious organisms, viruses, or fungi unless special media are used. They carry a significant risk of 'false positives' due to skin contamination, which leads to unnecessary antibiotic use and prolonged hospital stays. They are also 'slow', often taking 24-48 hours for initial growth.

High-yield: Always take cultures *before* the first dose of antibiotics in sepsis. Remember the Sepsis Six. Be aware of the 'Endocarditis' requirement for serial cultures (at least 3 sets). Understand that one bottle of 'Staph epidermidis' is likely a contaminant, but three bottles of the same is a likely prosthetic valve infection.