Positive Blood Cultures – convincing or contaminant? EM Reflections April 2022

Thanks to Dr. Joanna Middleton for leading this month’s discussions.

All cases are imaginary but highlight important learning points.

Authored & Copyedited by Dr. Mandy Peach

You are working an evening shift and a nurse hands you a printout of positive blood culture results for a patient seen previously in the department. You don’t recognize the name and ask for the chart to be pulled.

The results state:

Anaerobic vial: Gram positive bacilli suggestive of the genus Bacillus/Clostridium seen after 0.64 days incubation

What is your approach?

It’s concerning that there is growth so soon in a vial, but you don’t know the patient or story yet. Although your immediate thought is to bring the patient back in, the question is – how likely is this to be a contaminant?

North York General Blood Culture Algorithm provides a step by step approach to dealing with positive blood cultures. You review your case

  1. Are there two cultures showing growth of the same organism?
    1. No – so far just one vial
  2. Is it gram negative bacilli or yeast?
    1. No – it is Gram Positive
  3. Is it gram positive cocci in pairs or chains?
    1. No – it is bacilli

So far, these are more reassuring factors for potential contamination. At this point, patient risk factors come into consideration. You get the patient chart and review:

Patient risk factors associated with poor outcome include:

  • Immunocompromised (HIV, active chemo, active immunosuppresants, uncontrolled DM)
  • Internal hardware (artificial heart valves, pacemakers, joints)
  • Suspicion/history of endocarditis
  • Central line
  • MSK pain concerning for osteomyelitis/discitis
  • Age < 3 months

The chart describes a 70 yo male who presented with a likely syncopal episode NYD. On scene with EMS there was documented hypotension that improved with fluid resuscitation. Blood cultures were drawn as sepsis was on the differential for hypotension. The past medical history includes DLP, HTN and a L TKA for OA.

The TKA counts as internal hardware, therefore the patient has a risk factor for poor outcome. This patient is called back to the ED for repeat cultures and search for an alternative source.

What if this was a healthy 20 yo non-pregnant female with the same presentation who has no risk factors for poor outcome?

You can further risk stratify the bacteria.

Bacteria that are considered high or intermediate risk must be called back for reassessment and repeat cultures.

Blood cultures should include:

  • 2 sets from 2 peripheral sites collected at least 30 min apart
  • If considering endocarditis or fever of unknown origin consider taking an additional set.

High risk bacteria is unlikely to be contaminant

Intermediate risk bacterial could possibly be contaminant.

Regardless, once cultures are redrawn and the patient reassessed in the ED, discuss the case with IM or ID for management.

If the patient is healthy, unlikely to have true bacteremia based on organism isolated and reports feeling well on verbal reassessment over the phone – give good discharge advice on when to return to the ED and follow up on speciation results.

Feeling unwell? Return to ED for reassessment and repeat cultures, looking for an alternate source.

What features are concerning for true bacteremia?

If 2 vials are growing the same organism, or the organism is reported as gram negative or yeast it is most likely true bacteremia – call the patient back to the ED, redraw blood cultures, start appropriate antimicrobials or anti-fungal agents and admit to hospital.

If the patient grows gram positive cocci in pairs or chains this is also unlikely to be a contaminant with a 82% chance that it is true bacteremia – call the patient back to the ED, redraw blood cultures and start IV antibiotics: Vancomycin to cover MRSA and Ceftriaxone; admit to hospital.

Pen allergy? Cover with vancomycin alone.

We see S. epidermidis reported all the time – is this always a contaminant?

Although a common contaminant, in the setting of any hardware (even remote orthopedic procedures) this can be pathological. If it grows in 2 vials with a history of hardware, treat as true bacteremia. If growing in 2 vials and no hardware, still reassess the patient and obtain repeat cultures.




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