An ongoing dialogue on HIV/AIDS, infectious diseases,
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:
- Blood is collected in a sterile fashion.
- 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.
- In the micro lab, the tubes are centrifuged to concentrate any bugs that might be present.
- The sediment is aspirated and placed on appropriate culture media — e.g., fungal media for “fungal isolators” and mycobacterial media for “mycobacterial isolators.”
- 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There is a literature about the superiority of isolators over standard blood cultures, but these comparisons are mostly outdated — for example, with advances in blood culture technology (see video below), candida grows just fine in regular blood cultures today.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:
- 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.
- 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: