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.
Types of specimen
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
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
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.
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.
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.
- 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.
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.