Synopsis
Collect, transport and interpret microbiological specimens so that results answer a defined clinical question, distinguish contamination from infection and enable safe antimicrobial narrowing.
- Define the question before collecting: bloodstream infection, source pathogen, colonisation, resistance or treatment failure require different specimens.
- National adult default: obtain at least two blood-culture sets—two aerobic and two anaerobic bottles—using aseptic peripheral venepuncture, place 8–10 mL in each bottle and send them immediately so incubation can begin within 4 hours; verify the current local bottle system.
- Obtain cultures before antimicrobials when this causes no harmful delay, and record prior doses because partial treatment changes sensitivity and interpretation.
- Sample the actual site whenever safe: deep tissue or aspirate is usually more informative than a superficial swab from colonised skin.
- Label anatomical site, time and clinical context accurately, transport promptly and discuss unusual, hazardous or time-dependent organisms with microbiology before collection.
- Interpret results using organism identity, number and timing of positive bottles, source concordance, devices and host phenotype; not every isolate is a pathogen and not every negative sample excludes infection.
Key red flags
Continued growth of a significant organism despite treatment suggests endovascular infection, uncontrolled source, ineffective exposure or retained infected material and requires urgent review.
Suspected tuberculosis, viral haemorrhagic fever, brucellosis or other laboratory hazard requires advance communication so collection, packaging and processing protect staff.
Investigation priorities
Detect bacteraemia and provide identification and susceptibility results.
Management branches
Systemic infection, endovascular features or severe focal infection makes bacteraemia clinically plausible.
- Prepare the correct bottles, perform hand hygiene and skin antisepsis, then collect the adult default of at least two sets from separate peripheral venepunctures, filling every bottle with 8–10 mL and labelling exact times and sites; follow a syndrome-specific protocol when indicated.
- Obtain source samples in parallel, start indicated antimicrobials within the syndrome-specific timeframe and record every pretreatment dose.
- Review preliminary Gram stain immediately, adjust treatment only in clinical context and repeat cultures when organism or focus requires proof of clearance.