The title of this post describes a very basic concept, but one that is sometimes forgotten or hard to follow. The key point is that the goal of treating a sick pet is to make the pet well. Getting well and getting "normal" laboratory data aren’t necessarily the same thing.
A question that comes up a lot with MRSA, MRSP and other bacterial infections is "should my pet be tested after treatment to see if the bug is gone?" The answer is usually "no."
One reason to not re-test is just what I said above. The goal of treatment is to make the patient healthy. That doesn’t necessarily mean that MRSA, MRSP or whatever bug is causing the problem needs to disappear. In fact, the bacterium that causes a given infection often remains (in smaller numbers) in or on the body somewhere. If it’s a skin infection, the bacterium may still be present on the skin where the infection was. However, if the body is handling it well, then it’s not necessarily a problem. We have to remember that every animal (and person) has multiple types of bacteria in or on it that can cause disease given the right circumstances. We’re never going to eliminate them all. In fact, trying to get rid of all potentially bad bacteria would probably result in bigger problems.
Another concept that I emphasize a lot is only do a test if you have a plan for using the result. If you don’t have a clear reason to do the test, and if the result won’t impact your decisions or provide information you need in the future, then why do it? While a negative culture might be nice to see, it’s not necessarily a guarantee that a particular bug is gone. Furthermore, a positive culture doesn’t lead to actions that are any different from those that would be taken if the culture is negative in most situations because, as mentioned above, we’re looking for clinical cure (getting better) rather than microbiological cure (getting rid of the bug). Rarely would we extend treatment or do anything different in response to a positive culture in a healthy animal.
The bottom line is post-treatment cultures are rarely needed. There may be some circumstances where testing after treatment is useful and where the results would lead to a defined plan of action, but these are few and far between.
Image: A Mueller-Hinton agar culture plate being used to test the antibiotic susceptibility of a bacterial isolate according to the Kirby-Bauer method. (source: CDC Public Health Image Library #10785)