Blood Cultures

Question 1

What are blood cultures taken for?

Answer 1

• Diagnosis of bacteremia, fungaemia, infective endocarditis and pyrexia of unknown origin (PUO)
• Important in diagnosis of prosthetic device infections and intravascular line-associated sepsis
  Detect bloodstream infection associated with pneumonia, septic arthritis, osteomyelitis etc -> spread of disease to blood stream etc

Question 2

When is resistance seen in BSI

Answer 2

Seen in gram negatives

Question 3

When should blood cultures be taken?

Answer 3

Should be taken before starting antibiotics

If patient takin antimicrobials then the blood cultures should be collected just before the next dose is due I.e. when antimicrobial concentration is at its lowest

Question 4

How should blood cultures be taken?

Answer 4

• Two blood culture sets is recommended, with the second or third set being taken from a different site
  
  • Increases yield
    Allows recognition of any contaminants – organism present in one set but not the other etc

Question 5

When during an infection should blood cultures be taken and why?

Answer 5

• Samples should be taken as soon as possible after a fever spike
  
  • Bacteraemia is intermittent and is related to onset of fever and rigors which occur 30 to 60 mins after bloodstream invasion
    Timing is less important in continuous bacteraemia I.e. in infective endocarditis etc

Question 6

Talk about the blood culture bottles

Answer 6

• Two bottles, one aerobic and one anaerobic
  
  • Bottles contain tryptic soya broth + beads
• Approximately 7mls of blood is needed -> no more than 10mls as this will cause false positives
• Minimum delay between sample being taken and processing in lab
• Bottle stored at room temperature until being loaded onto analyser
  Specimens other than blood can be processed in the BC bottles where appropriate I.e. fluids from normally sterile sites etc

Question 7

How do we process blood cultures?

Answer 7

• Processed onboard the BacT/ALERT® VIRTUO™ Instrument
• Onboard bottles are incubated for 5 days
  
  • No extension of incubation needed for organisms such as brucella, for patients on antimicrobials, fungal infections, infective endocarditis or for fastidious or slow growing organisms
• Negative bottles will be automatically unloaded from the instrument into a waste bin and any negative results are transmitted over to the LIS
• Positive bottles will cause an audible alarm to sound and an indicator light to flash yellow
  
  • Positive bottles are unloaded by user request
  • Alarm and indicator will continue until positive bottles are removed from the instrument
    Positive results are transmitted over to the LIS

Question 8

How do you process a positive blood culture?

Answer 8

• Positives must be handled in the class II microbiology safety cabinet
• Disinfect the septum of the bottle using an alcohol wipe and allow to air dry
• Label up a blood agar, chocoalte agar, MacConkey agar and Mueller Hinton agar
• Wipe a glass slide with an alcohol wipe and allow to air dry
• Label glass slide
• Using a subculture unit, subculture 1-2 drops of blood onto the surface of the glass slide and each of the four labelled plates
• Streak out the plates and make a thin smear on the slide for gram staining
  Make sure to use a separate loop for each plate
• Allow the slide to dry and fix on the hotplate
• Perform a gram stain once dry
  
  • Gram must be allowed to fix for 20 minutes before staining
    This will lyse any red blood cells presen

Question 9

What plates are put up for blood cultures?

Answer 9

Blood agar inced anaerobically @ 37 degrees
Chocolate agar inced in 5-10% CO2 @ 37 degrees
MacConkey agar inced aerobically @ 37 degrees
MH inced aerobically @ 37 degrees

Question 10

What discs are put up for blood cultures?

Answer 10

Metronidazole is put up on blood agar

Question 11

What blood cultures do we put up a gram stain for?

Answer 11

All positive cultures will get a gram stain

Question 12

For how long should you fix a gram before staining and why?

Answer 12

Gram will need 20 minutes before staining to allow any rbcs to lyse

Question 13

What might cause a false negative blood culture?

Answer 13

• Underfilled blood culture bottles
• Patient on antibiotic treatment
• Fastidious or non-culturable organisms (especially after antimicrobial treatment)
  Blood culture has been incubated prior to going on analayser -> missed the bacterial growth phase, bacteria now in plateau or decline phase causing a false negative

Question 14

What might cause false positive blood cultures

Answer 14

• Overfilled blood culture bottle
  High white cell count

Question 15

What might cause a positive BC but negative subculture with a positive gram stain

Answer 15

S. pneumonia
Fastidious organisms

Question 16

Talk about S. pneumoniae blood cultures

Answer 16

Often not able to subculture
○ Densely grown cultures undergo autolysis or self-destruction
○ Bottles should be subculture as soon as possible after a positive flag is detected to avoid this

Question 17

Talk about fastidious organisms in BCs

Answer 17

○ Additional or supplemented media, prolonged incubation or alternative growth atmosphere should be considered
○ E.g. Campylobacter species, Helicobacter species, Capnophilic organisms and slow-growing anaerobes

Question 18

What would you do with a positive BC, negative gram and negative culture

Answer 18

• Cytospin to make a more condensed slide to gram stain
  ○ If still gram stain negative then re-load the blood culture onto the same machine
  You are trying to condense down anything that is there -> organism may need more time to grow in blood culture bottle before being able to culture etc -> could also just be a true false positive

Question 19

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