Skip to main content
A Positive PCR Test May Not Mean Positively Sick

You are listening to Health Library:

A Positive PCR Test May Not Mean Positively Sick

Aug 05, 2015

PCR-based diagnostic tests are becoming increasingly common in clinical settings because they are much more sensitive and accurate than older tests, and provide results within hours, not days. Yet a study published in Clinical Infectious Diseases shows that half of those who tested positive for respiratory viruses did not report feeling ill. These findings suggest that some test results should be interpreted with caution. Co-first author Dr. Carrie Byington, professor of pediatrics and co-director of the Utah Center for Clinical and Translational Science, talks about the results and how clinicians can use the information to improve patient care. Learn more.

Episode Transcript

Interviewer: A study shows that half the time, a positive test does not mean positively sick. Up next on The Scope.

Announcer: Examining the latest research and telling you about the latest breakthroughs, the Science and Research Show is on The Scope.

Interviewer: I'm talking with Dr. Carrie Byington, Professor of Pediatrics at the University of Utah and Co-Director of the Utah Center for a Clinical and Translational Science. Dr. Byington, you've published a study in the journal, "Clinical Infectious Disease," and as a part of this study, you turned up some pretty surprising results about how often infections with respiratory virus actually cause illness. First of all, what do we mean by that?

Dr. Byington: The study that we performed used a very sensitive diagnostic test called PCR and PCR picks up nucleic acid, like DNA and RNA, from different viruses and bacteria. What we know is that it's very sensitive and can pick up these materials very frequently, but as clinicians, sometimes it's difficult for us to interpret a positive test result. We really didn't know how often would we see positive results in individuals who have no symptoms. Like, if I just tested you on a day that you were well, might I find a virus in your nose?

So we did a study over an entire year where we followed families every week and collected samples from them every week, whether or not they were sick. So some days they were sick and some days they were well when we collected the samples. And we also had them fill out symptoms diaries that would tell us had they been sick, did they have a fever, were they having cold symptoms during the time, the week of collection.

So when we were able to put all the data together and to analyze it, we found that about half of the time that we were able to detect a virus in the nose of an individual, they had no symptoms. And so that is really striking to us. It tells us that the virus, at least the viral products of DNA and RNA, their nucleic acid, can stay in our noses for some time. And as a clinician, that information is really important to me when I am faced with a sick patient. So when I'm faced with a sick patient and I have a positive test for a virus, how likely is it that I can attribute the illness to the virus that I found in the nose?

Interviewer: Well, that's interesting. I mean, it kind of calls into question how useful that sort of test is.

Dr. Byington: Well, first of all, I think that having the test is really beneficial and they've been demonstrated to help in the treatment of patients in multiple settings, not just in our study or in Utah, but really all over the world. One of the things that our study will help people to see is that it isn't just 50-50 across the board. It really depends on how old you are and younger children, especially infants and children younger than 5, are much more likely to have symptoms with their viral infection than older adults. So that, as a pediatrician, is useful information to me.
The other thing that was very helpful is, we followed people when they were sick and well and so we were able to see someone acquire a virus and have illness symptoms and then, follow them over the weeks to see how long was it till their symptoms went away and how long was it until their test became negative. And we often see that the symptoms go away before the test becomes negative. And we also saw that different kinds of viruses have different risks for producing symptoms.

So a virus, like influenza, very often produces symptoms, which I think was not terribly surprising. All of us know that influenza is a very serious infection and can lead people to be hospitalized. On the other hand, rhinovirus, which is an extremely common virus and the cause of the common cold, very rarely did adults have symptoms when they had rhinovirus. And even in children, young children, it was only about half the time that they had symptoms with rhinovirus.

So again, as a clinician, if I see a child who is ill, especially a child who is very ill, and I have a positive test for rhinovirus and that's all that I have, I'm going to be suspicious that that rhinovirus is not the cause of the illness that I see before me and that I'm going to need to look for other things to make sure that I'm not missing another potentially treatable infection.

Interviewer: And you had mentioned a really interesting point, which is that someone's symptoms might go away before the virus goes away. Does that mean at that trailing off period that they're still spreading the virus or we don't know?

Dr. Byington: We don't know for sure, but we suspect that they may be able to. So that is also really, really important for a number of reasons. It helps inform us about how we would isolate a patient if they came into the hospital because even if they don't have symptoms, they may be able to spread a virus to someone who is immunocompromised or someone who could become very ill if they were exposed to the virus. So that's really important.

It's also important to us as we think about our healthcare workers, how do we make sure our healthcare workers, who are exposed to so many things, number one, stay healthy, and number two, that we can protect the patients that they are caring for and making sure that our healthcare workers are not spreading the viruses in the hospital setting.

Interviewer: If healthcare providers can kind of understand these new answers between when symptoms arise for each type of virus that might help them to interpret the results that they get in a clinical setting.

Dr. Byington: Well, when you're faced as a clinician with a sick individual, you have to make decisions and you have to make decisions with the best data that you have. You don't always have all the data and you don't always have all the research studies to inform. And so that's one of the reasons this was an important problem that we were encountering every day in our practice and we really wanted to address it.

Announcer: Interesting, informative and all in the name of better health. This is The Scope Health Sciences Radio.