May 21st, 2013

ID Learning Unit — “Isolator” Blood Cultures

Here’s a little secret about those brilliant ID consults we do on patients with mysterious fevers:

Sometimes we don’t know what’s going on either.

I know, I know — shocking.

But now that the secret is out, I can tell you something we do know, and that’s how to recommend lots of tests — the more obscure, the better. Including a particular favorite, isolator blood cultures. If you do inpatient medicine, chances are you’ve had an ID consultant recommend these and may even have ordered them without knowing precisely what they are, or more importantly, or how they differ from the regular blood cultures.

So here’s the story on isolator blood cultures — “isolators” for short — almost in plain English:

  1. Blood is collected in a sterile fashion.
  2. Instead of going into regular blood culture bottles, the blood is put into special tubes containing a chemical that lyses (explodes) both red and white blood cells, “releasing” intracellular organisms.
  3. In the micro lab, the tubes are centrifuged to concentrate any bugs that might be present.
  4. The sediment is aspirated and placed on appropriate culture media — e.g., fungal media for “fungal isolators” and mycobacterial media for “mycobacterial isolators.”
  5. Wait. Potentially for a very long time (weeks).

These cultures are also known as “lysis-centrifugation” cultures, which is more descriptive than “isolators” but harder to say.

So when should you order them? There is a literature about the superiority of isolators over standard blood cultures, but these comparisons are mostly outdated — for example, with advances in blood culture technology (see video below), candida grows just fine in regular blood cultures today.

Furthermore, even though isolators may be superior to standard blood cultures for certain rare infections (e.g., histoplasmosis, bartonella, blastomycosis), in most (all?) of these conditions, use of antigen, serology, or PCR testing has supplanted culture methods entirely.

So that leaves one proven and one possible remaining indication for isolator blood cultures:

  1. Proven: Diagnosis of disseminated mycobacterial infection (in particular, M. avium complex) in a patient with advanced AIDS or other severe immunodeficiency. Regular blood cultures are pretty much useless here.
  2. Possible: Diagnosis of some bizarre, fastidious pathogen (e.g., Malassezia furfur) in a patient with culture-negative endocarditis or a vascular line-related infection.

Which means that most of the time, when your ID consultants recommend isolator blood cultures, you can ignore them.

And show them this video by a Dr. Kimmitt, who certainly knows her stuff but clearly is in no joking mood:

2 Responses to “ID Learning Unit — “Isolator” Blood Cultures”

  1. Wissam Atrouni says:

    You mean regular bacterial blood cultures are useless for disseminated MAI. How about AFB blood cultures compared to lysis- centrifugation blood cultures for MAI? Another pathogen that comes to mind Brucella. Do you need lysis centrifugation to that one?

  2. Elliott Wolfe, MD says:

    Dr. Sax,

    Your explanation with Dr. Kimmitt’s “no-joking” video are an outstanding teaching moment for all. I compared Dr. Kimmitt’s precise description with methods I used during my bacteriology courses at UCLA in ~1953; of course, the traditional plating methods were as accurate then as today. During my internship at NYC’s Mt. Sinai and other residency venues, members of the house staff did initial Gram staining and plating of cultures from sputum, urine, wounds, abscesses, and, even, CSF. We often identified organisms based on Gram stain and the subsequent appearance of bacteria grown on cultures, as early as 8 hours; incubators were on the wards and checked frequently, even during the early morning hours — because we were there! (We even could create high CO2 environments for cultures.) Blood cultures were immediately placed in the ward incubators and periodically retrieved by laboratory associate; of course, we identified growth crudely, by external inspection.

    BTW, Dr. Kimmit’s video should be distributed to all medical schools and placed on YouTube.

    Thank you for your continuing excellent blog and ID advice.

    Elliott Wolfe, MD, FACP

HIV Information: Author Paul Sax, M.D.

Paul E. Sax, MD

Contributing Editor

NEJM Journal Watch
Infectious Diseases

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