Additional Concerns for Identifying C. Difficile Infection
MAY 15, 2018
MD Magazine Staff
Peter Salgo, MD: What do the guidelines say about multiplex PCR [polymerase chain reaction]?
Dale N. Gerding, MD: Well, the guidelines say nothing.
Peter Salgo, MD: Oh, thanks so much for that.
Dale N. Gerding, MD: Multiplex PCR testing is a relatively recent development. It’s a PCR that covers multiple diarrheal pathogens. We are seeing, in our limited experience, more positive tests with multiplex PCR for Clostridium difficile [C. diff] than we have seen with specific PCR testing for C. diff. We don’t understand what exactly is going on. This is so new that we did not feel we could cover it in the guidelines. We don’t really understand what is going on. But, we think this is going to create some issues around diagnosing C. diff.
Peter Salgo, MD: It sounds like an attractive option, right? You send off 1 test, this multiplex PCR, and it’s going to come back and tell me exactly what’s wrong.
Darrell S. Pardi, MD: The problem with that is, some people use that as an excuse not to think. It goes back to Yoav’s comment on the diagnostic algorithm for C. diff. Your interpretation starts with your pretest probability. So, you need to think about the patient before you order your test. The multiplex PCR has upwards of 20 different pathogens. For many of them, there is no treatment. Some of them don’t need treatment. Some of them get worse with treatment. So, you should think about it before you order that test. Our lab says that if you’re going to order 3 individual tests or more, then it’s cost-effective to do the multiplex. But, if you really only want to know about the C. diff, just order the C. diff test.
Peter Salgo, MD: So, it’s not just a “blast away” screening test?
Darrell S. Pardi, MD: It’s used that way.
Dale N. Gerding, MD: The studies in the past, when diarrhea occurred in a hospital setting, in someone who was in the hospital for more than 2 days with a new onset of unexplained diarrhea, and you tested traditionally for Staphylococcus aureus, Salmonella, or Shigella, common diarrheal pathogens, you never found it. The only thing you found was C. diff. So, why do you need a multiplex test in that setting? Basically, you want to specifically test for C. diff in that hospital setting. Now, if you’re out in the community, that’s a different story. There, those other pathogens are found quite often.
Yoav Golan, MD: I would caution against using C. diff as part of the multiplex. The multiplex test is actually very good. You may have Shigella, Salmonella, rotavirus, or norovirus. It’s really hard to differentiate them. First of all, you’re going to have 2 different C. diff tests in your lab. Secondly, as some work has shown, including work done by Dale, that colonization with C. diff increases with the longer stay in the hospital. And, if you’re going to use indiscriminate tests such as that, you’re going to identify a lot of colonized patients. Therefore, in our hospital, for example, we chose our multiplex test. We actually changed the nature of the test, as compared to all of the other PCR tests that we’re using. We didn’t want to have C. diff as part of our multiplex. Or, if you do have one, I would suggest that you consider not reporting C. diff every time the multiplex test is being sent.
Peter Salgo, MD: All right. And lastly, since you brought this up, how common is it to have an asymptomatic carrier? Somebody who you’ve sent this thing off on, who is asymptomatic, in whom you’ve sent it and they’ve come back positive, how big of a problem is it?
Yoav Golan, MD: Maybe Dale should talk about this study.
Dale N. Gerding, MD: The carrier rate in the hospitals is probably 3 to 5 times higher than the C. diff infection rate. Many more patients pick up this organism while hospitalized. In one study that we did, they were picking it up at a rate of 7% per week. When we did the study, the length of stay was much longer. In our study, 20% of patients were colonized after they had been in the hospital for 3 weeks.
Peter Salgo, MD: Twenty percent?
Dale N. Gerding, MD: Yes. This is 3 weeks in the hospital. Now, hardly anybody stays in the hospital for that long. Still…
Yoav Golan, MD: Well, people stay in nursing homes and rehabilitation centers. For that purpose, it’s like a hospital.
Dale N. Gerding, MD: In a nursing home setting, you can find the colonization rates to be that high as well. Now, if they stay away from antibiotics, this is transient. This goes away. But, when patients are colonized and they get diarrhea for some other reason, the testing will still show C. diff.
Peter Salgo, MD: This goes back to what you were discussing—clinical guidelines. Somebody’s got to have a high end of suspicion. It might be C. diff. Then, you go look for it. If they don’t have diarrhea, and if they don’t have anything else, and you get back a C. diff, it’s not C. diff.
Yoav Golan, MD: And, if you load them with lactulose and they develop diarrhea at some point in time, it’s just the same.
Peter Salgo, MD: Stop. I remember, when I was taught to use lactulose, that you push to diarrhea because that’s how it works.
Transcript edited for clarity.