Blood Culture Diagnosis

A blood culture tests a patient's blood for living bacteria by allowing any organisms present to multiply in a nutrient medium. Recovery and identification of a clinically significant organism from normally sterile blood provide direct evidence of bacteraemia and establish an aetiological diagnosis, subject to exclusion of contamination.

Collection of the specimen

Blood is collected aseptically into culture bottles containing nutrient broth, preferably before antibiotic therapy begins. Adequate blood volume and proper skin antisepsis are essential because low organism numbers reduce detection, while skin flora may cause contamination.

• Separate culture sets from different venepuncture sites help distinguish a genuine bloodstream infection from contamination.
• Clinical details and recent antimicrobial exposure must accompany the specimen because antibiotics can reduce culture yield.

Detection and identification

Culture bottles are incubated so that viable bacteria can multiply. Automated systems commonly detect microbial growth through changes associated with metabolism, after which a Gram stain and subculture on solid media guide identification.

• Colony appearance, microscopy and biochemical or validated instrumental methods establish the organism's identity.
• Growth proves that viable organisms were present in the sampled blood, rather than merely detecting bacterial genetic material or an immune response.

Clinical interpretation and susceptibility testing

An isolate is interpreted using the organism identified, the number of positive culture sets, the clinical picture and the pattern of positivity. Antimicrobial susceptibility testing then determines whether the isolate is susceptible or resistant to tested drugs and supports targeted treatment.

• A recognised pathogen repeatedly recovered from separate samples strongly supports true infection.
• Common skin organisms growing in only one sample may represent contamination rather than disease.
• A negative blood culture does not exclude infection, particularly after antibiotics, with low-level or intermittent bacteraemia, or when the infection remains localised.