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'D-Zone' clue is key to MRSA clindamycin resistance: lab test for inducible resistance.

ASPEN, COLO. -- Physicians should make sure their microbiology lab is testing for both inducible and constitutive resistance to clindamycin when they send samples for susceptibility testing, Dr. Carol J. Baker said at a conference on pediatric infectious diseases sponsored by Children's Hospital, Denver.

With the rise in methicillin-resistant Staphylococcus aureus (MRSA), susceptibility testing has become routine.

But unless the lab takes special care, the report may say that the patient's S. aureus is susceptible to clindamycin when actually it is not.

In constitutive resistance, the microbe is clearly resistant to both erythromycin and clindamycin, and the lab will be able to pick this up using any susceptibility test, Dr. Baker explained at the meeting, which was also sponsored by the University of Colorado.

The problem is with inducible resistance.

During ordinary testing, the microbe appears to be resistant to erythromycin and susceptible to clindamycin, but once the patient is exposed to clindamycin, he or she quickly develops resistance. "It's very clinically important," said Dr. Baker, who is head of the pediatric infectious diseases section at the Baylor College of Medicine in Houston.

"You need to know about this or your patients are going to have problems.... If you think you're treating [the infection] and all of a sudden your patient has a recurrence of pain and fever, it's a great disappointment, to say the least," she commented.

Fortunately, it's quite simple and inexpensive to test for inducible resistance. The lab merely has to place erythromycin near clindamycin on the culture dish.

Normally there will be a clear circular area around the clindamycin sample, which indicates susceptibility, and no such area around erythromycin, which indicates resistance.

Sometimes, however, the circular area around the clindamycin will have a flat or blunted edge where that sample is adjacent to the erythromycin sample, and that's proof of inducible resistance to clindamycin.

The flat edge on one side of the circular area around the clindamycin sample looks something like the letter D, and therefore Dr. Baker refers to this as the "D-zone."

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Title Annotation:methicillin-resistant Staphylococcus aureus; Infectious Diseases
Author:Finn, Robert
Publication:Pediatric News
Article Type:Brief Article
Geographic Code:1USA
Date:Sep 1, 2003
Words:337
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