Paper I
Question
Describe the indications and different methods of blood culture. Discuss sample collection for blood culture. (3+2+3)
Answer
Indications for blood culture: suspected bacteraemia/septicaemia (unexplained fever, especially with rigors); suspected infective endocarditis; suspected enteric fever (typhoid, particularly in the first week); fever of unknown origin; suspected catheter-related bloodstream infection; sepsis work-up in critically ill patients; and fever in a neutropenic/immunocompromised patient.
Methods of blood culture:
- Conventional (manual) broth culture — blood is inoculated into a nutrient broth (e.g., brain-heart infusion broth, or specific bile broth for suspected typhoid, which neutralizes serum bactericidal activity), incubated at 37°C, and periodically subcultured onto solid media to detect growth; slow, labour-intensive.
- Biphasic (Castaneda) method — a bottle containing both a liquid broth and a slanted solid agar surface; the bottle is simply tilted periodically to flood the agar with broth, allowing subculture without opening the bottle (reducing contamination risk) — historically valuable for Brucella and typhoid blood cultures.
- Automated continuous-monitoring blood culture systems (e.g., BACTEC, BacT/ALERT) — the current standard in most modern laboratories; bottles are continuously monitored for microbial growth (via CO2 production detected optically/colorimetrically), providing much faster detection (often within hours to 1–2 days) than manual methods, and can be linked to identification/susceptibility testing systems.
- Lysis-centrifugation method — blood is lysed to release intracellular organisms, then centrifuged and the sediment plated directly; particularly useful for fastidious/intracellular organisms and fungi.
Sample collection for blood culture:
- Timing: ideally collected before starting antibiotics, or at the time of a fever spike/rigor when bacteraemia is most likely to be detectable.
- Volume: an adequate blood volume is critical for sensitivity (typically 8–10 mL per bottle in adults, less in children), since bacteraemia often involves low numbers of organisms per mL of blood.
- Number of sets: 2–3 sets from separate venepuncture sites, ideally spaced apart in time, improves sensitivity and helps distinguish true bacteraemia from contamination (a true pathogen typically grows in multiple sets, while a contaminant — e.g., skin flora — typically grows in only one).
- Aseptic technique: rigorous skin antisepsis at the venepuncture site (e.g., chlorhexidine or iodine-based skin preparation) is essential to minimize contamination by skin commensals (e.g., coagulase-negative staphylococci), a common cause of false-positive results.
- Bottle types: separate aerobic and anaerobic bottles are typically inoculated to cover the full range of possible causative organisms.
- Prompt transport to the laboratory for timely incubation.