Blood culture

Keywords: Culture blood, BC

Specimen:

The volume of blood collected (min 20 mL in adults) is a critical factor in determining positivity ie, more blood collected increases the likelihood of a positive result in a bacteraemic patient.

Method:

Culture in enriched non-selective broth for aerobes, anaerobes and yeasts.

Incubation for 5-7 days, often extended to 14-21 days for suspected bacterial endocarditis or infection with Brucella spp or yeasts, although there is evidence that prolonged incubation is unnecessary in suspected bacterial endocarditis.

Growth is detected by the presence of turbidity or haemolysis, Gram stain, or more commonly production of carbon dioxide, alteration in redox potential or change in pH detected by an automated monitoring system.

The organism is identified by Gram stain of the broth, colony appearance after subculture to solid media and biochemical and/or antigen tests.

Application:

To establish the diagnosis in suspected Septicaemia, Endocarditis (infective), Pericarditis, Osteomyelitis, bacterial meningitis, septic arthritis, pyelonephritis, enteric fever.

To identify the causative organism(s) in severe Pneumonia, Post-partum fever, Pelvic inflammatory disease, cannula sepsis; neonatal epiglottitis and sepsis. Investigation of patients with PUO.

Interpretation:

After growth has been detected in the broth the Gram stain appearance guides the initial choice of antibiotics (which may require modification after identification and antibiotic susceptibility testing).

Organisms usually regarded as non-pathogenic contaminants may cause infection in individuals whose defences are compromised by immunosuppression, trauma, or the presence of a prosthesis or an indwelling line.

Most pathogens grow within 1-2 days; some organisms may require longer eg, Candida spp, Brucella spp.

Cultures are usually positive in Endocarditis (infective) (if collected prior to antibiotic treatment) and early typhoid.

Negative cultures do not exclude infection.

Reference:

Strand CL, Shulman JA. Bloodstream Infections. Laboratory Detection and Clinical Considerations. Chicago: ASCP Press, 1984.

Weinstein MP. J Clin Infect Dis 1996; 23: 40-46.