Blood Culture

Question 1

Describe the aim of a blood culture?

Answer 1

To detect the presence of bacteria or fungi in the blood

• to identify the type present
• to guide treatment

Question 2

List some indications for ordering a blood culture?

Answer 2

Indications:
• Diagnosis of presentation of fever, tachycardia or hypotension with/without suspected focus of infection
• E.g. suspected septicaemia, endocarditis, bacterial meningitis, septic arthritis, osteomyelitis, pyelonephritis, enteric fever, FUO
• Threshold lower in immunocompromised pts

Question 3

Describe the bottles used in specimen collection for a blood culture?

Answer 3

Bottle contents:
• Both aerobic and anaerobic bottles: contain growth factors (soybean and casein broth, yeast extract, sucrose, pyridoxal, bicarbonates) and 0.05% sodium polyanetholesulfate (SPS) resin beaks (to inactivate antibiotics).
• Anearobic tube: reducing agent (glucose) and thioglycollate (consumes O2) -> aids anaerobic bacteria growth

Note: SPS also inhibits some Neisseriae species, anaerobic, gram positive cocci and Gardnerella vaginalis
- If suspected, alternative medium (sans SPS) should be used.

Question 4

Describe the process of specimen collection for a blood culture?

Answer 4

Specimen collection
• Obtain 2-3 samples from different sites over 1hr
- Note: infective endocarditis samples taken over 24hrs
• Take samples PRIOR to antibiotic therapy OR ASAP if post emergency empirical therapy (from peripheral sites, less likely to cause false positives)
• Check bottle expiry date and label pt details
• Venipuncture with aseptic technique
• Clean rubber seal top w alcohol and dry
• Add 10mL to AEROBIC vile first (to stop air entering anerobic vile)
• Take specimens to lab promptly

Question 5

Describe the analysis of a blood culture sample?

Answer 5

Monitoring growth:
• Bottles incubated for 5-7 days at body temp
• If microorganisms present -> metabolise nutrients to release CO2 -> pH change -> subsequent disc colour change (containing fluorescent dye, at bottle bottom)
• Reaction modulates amount of light absorbed by flourescent material sensory -> alerts lab tech organisms present
• Malditof instrument can detect bacteria strain
• Preliminary results reported in 48hrs
• If no evidence of growth, samples discarded in 5 days
- Extended incubation done for Brucellosis or infective endocarditis

•If results positive -> subcultures indicated for sensitivities and identification

Question 6

What further subculture tests can be conducted once the presence of bacterial growth has been confirmed?n

Answer 6

1) haemolysis testing -> strep species
2) catalase testing -> staph positive, step negative
3) optochin testing -> strep pneumoniae sensitive
4) sensitivity testing (disc diffusion testing)- normal zone of inhibition 6mm
5) coagulase test -> staph aureus positive

Question 7

Describe the subculture results you would obtain from a strep pneumoniae culture?

Answer 7

Strep pneumonia:
o Gram-positive streptococci
o Facultative anaerobe
o Catalase negative
o Alpha-haemolytic bacterium
o Optichin sensitive