Blood Culture test

Blood Culture test

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An etiological diagnosis of bacteremia by aerobic and anaerobic cultivation of the blood, with identification and susceptibility test of the isolated organism(s). Blood culture should be made for cases with suspected septicemia, endocarditis, and bacteremia secondary to localized infections (pneumonia, intraabdominal abscesses, pyelonephritis, epiglottitis, and meningitis). In this case the blood culture may provide an etiological diagnosis of the localized infection.

Blood Culture test

Types of specimen

Whole blood

Criteria of specimen rejection

Blood collected in tubes or bottles other than aerobic and anaerobic blood culture bottles. If the information on the label does not match that of the request form. Specimens for anaerobic blood culture received in aerobic bottles or vice versa.


Blood is a sterile body fluid and normally contains commensals

Common pathogens

Streptococcus spp                Bacteroides fragilis and other anaerobic bacteria

Staphylococcus aureus        Coagulase negative staphylococci

Listeria monocytogenes                       Enteric gram negative bacilli

Corynebacterium jeikeium                  Neisseria meningitides

Haemophilus influenza                       Non fermenter gram negative bacilli

Salmonella typhi                                 Pseudomonas aeruginosa


Candida albicans                                 Cryptococcus neoformans

Coccidoides immitis                            Histoplasma capsulatum

Patient preparing

The major difficulty in interpretation of blood cultures is potential contamination by skin flora. This difficulty can be markedly reduced by careful attention to the details of skin preparation and antisepsis prior to collection of the specimen.

Skin preparation:

First cleanse the vein puncture site with isopropanol. Then use tincture of iodine or povidone iodine to disinfect the site using progressively larger concentric circles. Iodine should remain in contact with skin for about 1 minute or until dry to ensure disinfection. The vein puncture site must not be palpated after preparation. Blood is then drawn. Following vein puncture, alcohol is used to remove the iodine from the site.


Specimen collection

Blood cultures should be drawn prior to initiation of antimicrobial therapy. If more than one culture is ordered, the specimens should be drawn separately at no less than 30 minutes apart to rule out the possibility of transient bacteremia by self-manipulation by the patient of mucous membranes in the mouth caused by brushing teeth, etc. or by local irritations caused by scratching of the skin

Quantity of specimen

  • Children below 2 years: I mL of venous blood in 2 bottles
  • Children 2-5 years: 2 mL of venous blood in 4 bottles
  • Children 6-10 years: 3 mL of venous blood in 4 bottles
  • Children 11-15 years: 5 mL of venous blood in 4 bottles
  • Children above 15 years and adults: 5 mL venous blood in three sets of bottles (6 bottles).

Storage: Pre-incubate or maintain specimen at room temperature. Do not refrigerate

Container: One aerobic and one anaerobic blood culture bottle. Do not vent.

Specimen processing


  • Aerobic Blood culture bottle
  • Anaerobic Blood culture bottle
  • MacConkey Agar
  • Blood Agar
  • Chocolate Agar


Blood is injected to both aerobic and anaerobic bottles and incubated for up to 10 days at 37°C. Discard as negative after the 10 days incubation period is expired. During the incubation period, a gram stain and subculture onto appropriate media should be done

Post specimen processing

Interfering factors: Patient on antibiotic therapy

Result reporting: Any isolated organism will be reported. Antibiotic sensitivity will also be included with the report.

Turnaround time

Initial blood culture results will be reported as soon as it shows growth. Final results with sensitivity will be issued after 24-48 hours of the initial report. Negative results will be issued after 10 days of culture submission.

Interpretation of Positive Blood Cultures

  • Virtually any organism, including normal flora, can cause bacteremia
  • A negative culture result does not necessarily rule out bacteremia; false-negative results occur when pathogens fail to grow
  • A positive culture result does not necessarily indicate bacteremia; false-positive results occur when contaminants grow.
  • Gram-negative bacilli, anaerobes, and fungi should be considered pathogens until proven otherwise.
  • The most difficult interpretation problem is to determine whether an organism that is usually considered normal skin flora is a true pathogen.
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