MRSA or not?

MSVOYAGEUR msvoyageur at
Sat Sep 7 03:38:37 EST 1996

In our lab we have on occasion seen failure of the MRSA screening plate
after storage for one to three weeks, but prior to the expiration date. 
The commercial media company has confirmed this.  In order to have
confidence in our screening plate, we run known sensitive and resistant
strains along with patient purity checks.  This may solve the problem. 
Also, our automated MIC instrument will occasionally call a staph falsely
resistant to oxacillin.  A tip off is when the organism follows the
typical sensitivity pattern except for oxacillin, and so by default the
penicillins and cephalosporins.  These sensitivities are rechecked in our
lab with the standard KirbyBauer technique incubated a full 24 hours. 
Almost always the oxacillin is sensitive.  Hope this helps.  (While it's
my understanding the VRE screening plate is confirmatory, I don't believe
the MRSA plate can be confirmatory when it is in conflict with other
methods.)  Good luck.  Jan

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