June 15th, 2012

ID Learning Unit — The D Test

I suppose it’s not surprising that we’d follow-up the Etest with the D test, though perhaps if I were being alphabetical, the order would have been reversed.

The D test is important, because it screens for a form of clindamycin resistance in MRSA that might otherwise not be detected — the “inducible” kind, which can be associated with treatment failures. About half of MRSA isolates have this form of resistance.

Here’s how it works:

  1. Take some erythromycin-resistant, clindamycin-“sensitive” MRSA, spread it on a culture plate
  2. Drop an erythromycin disk on the left side of the plate, and a clindamycin disk on the right
  3. If there’s a flattening of the zone of inhibition between the two disks, then the test is positive, confirming inducible clindamycin resistance
  4. Report that bug as clindamycin resistant

First, critical thinkers will wonder why this is important, since we obviously don’t give erythromycin with clindamycin to an actual patient. My big-picture explanation is that it’s a marker for easily inducible resistance even without the erythro being present.

And second, note the critical step of putting the clindamycin disk on the right — otherwise the shape of the zone of inhibition won’t be a “D”, and everyone will be confused because who knows what to call that shape.

(That second part was a joke.)




One Response to “ID Learning Unit — The D Test”

  1. NA says:

    So I have to bite. Erythromycin induces the rRNA methylase, and I’m assuming maybe other macrolides do as well, but what are the other drugs/factors we are worried about that may cause the inducible resistance and thus clinical failure?

HIV Information: Author Paul Sax, M.D.

Paul E. Sax, MD

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NEJM Journal Watch
Infectious Diseases

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