Thursday, July 26, 2012

There are no MRSA

Several years ago I gave a presentation in which I made the point that “there are no Methicillin Resistant Staphylococcus aureus (MRSA), nor Vancomycin Resistant Enterococci (VRE) nor indeed any of the so called “superbugs” (better known as antibiotic resistant bacteria … UNTIL … a sample has been collected and transported to a laboratory capable to accurately testing for antibiotic susceptibility or resistance.  

I can and do argue that susceptibility testing is one of the most clinically significant procedures done by the microbiology laboratory.  In the absence of a laboratory a clinician can diagnose urine infection, or pneumonia, or abscess formation, or sexually transmitted infection, and can make a reasonably informed decision about treatment.  But definitive decision making about treatment of infection requires accurate information about likelihood of response.  And that means having access to accurate susceptibility testing results.

In most people, antibiotic susceptibility only needs to be “within the ball-park” because even if the bacteria are only partially damaged by a less-than-optimal agent, most healthy person’s neutrophils and antibodies will finish off the rest, and the infection goes away.  But in people with damaged immune response, susceptibility testing has to be far more accurate because successful therapy is far more dependent on drug activity.  

That is a long preamble to get to a simple point; antibiotic susceptibility testing must be monitored by regular and ongoing proficiency testing to ensure that systemic error has not been introduced in a way that could affect testing outcome and people’s lives.  

Fortunately we do that.  

Of all the tests that are performed by North American microbiology laboratories, antibiotic susceptibility testing is the one with the greatest degree of homogeneity.  Laboratories as a group usually do consistently well, and as such the percent achieved graph is much tighter.  Despite this there is a long tail with some laboratories with considerable under performance.  

I think I can understand and interpret this pattern.  Many, perhaps most microbiology laboratories use automated equipment that performs bacterial identification and susceptibility testing.  The susceptibility testing is very simple to perform and interpreted by software algorithms (so called "expert routines").  The amount done by the traditional manual agar diffusion assay is limited.  So the number of opportunities for error are reduced, unless the equipment is not working properly, or a slow-growing or special nutrient requiring bacteria is being tested.  

Consistent with most of our examination of percent achieved scores the tail of reduced performance is with the smaller laboratories (data not shown).  My bias is that the reduced performance is tied to  older equipment or to people not recognizing that unusual organisms may have questionable results and need some form of verification.  I suspect that both of these are directly linked to the problems of reduced supervision, and of reduced opportunities for continuing education and reduced budgets.  This is not a worker problem, but a management and supra-management problem. 

So here is the bottom line.  Susceptibility testing is done very well, but some laboratories continue to have problems.  Accurate information on the amount of antibiotic-resistance activity in a community is directly dependent on the Quality of the laboratory generating the information.  It is inappropriate to tolerate any laboratory getting their proficiency testing right only 70 or 80 percent of the time, or less.  It is a dis-service for clinicians and it puts patients at risk.

I mentioned that susceptibility is “one of the most clinically significant procedures” performed by microbiology laboratories.  The most significant procedures is the detection and documentation of cluster outbreaks either in the hospital or community setting.  

More on this later.

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