Diagnosing COVID-19 vs Flu


Differentiating between COVID-19 and influenza.


Peter Salgo, MD: We’re going to have a challenging season coming up. Let’s call it a respiratory season, for lack of a better word. We’ve got COVID-19, and we’ve potentially got influenza. For the sake of setting the table here, let’s run down the symptoms of COVID-19, contrast them with the symptoms of influenza, and see if we can parse some of this out. What are the common symptoms? What are the different symptoms? Then we’ll go on from there and figure out how to test for these things. George, do you want to start us off?

George Loukatos, MD: Sure. One of the biggest challenges we have is differentiating COVID-19 from the flu. That’s why we’re testing so many people for both, because the symptoms are fairly confluent through both disease processes. Outside of the unique things, like a loss of sense of smell and taste that ring the bell in my mind that I need to be thinking COVID-19, the symptoms are pretty similar across the board. It could be anything from mild fatigue, myalgias, runny nose, sore throat, headache, or you name it. But those are also the same symptoms of the flu. We have to test to differentiate between both.

To us, the biggest thing has been deciding what type of testing modalities we’re going to use. What’s going to be expensive vs inexpensive for the patient? What’s going to be accurate? What are my sensitivity and specificity rates with these different tests? We’ve spent a good bit of time trying to come up with our own protocols as to how we’re going to get accurate diagnoses and treatment plans in place.

Peter Salgo, MD: If you have somebody coming into the office and that person says, “I’ve lost my sense of taste and smell, and I’ve had some fevers, chills, and a cough,” it’s pretty clear you’re going to test for COVID-19. But if somebody comes in and says, “I’ve got a fever, I’m a little short of breath, and I’ve got a cough,” and you test for COVID-19, but it’s the flu, you’ve lost time that could be valuable. Similarly, if you test for the flu and it’s COVID-19, you’ve also lost time. Is it better to go ahead and test everybody for both? Does that make sense? Does anybody want to jump on that? What do you think?

George Loukatos, MD: I would do both.You just need to choose which modality you’re going to use. We’ve got swabs that are $10 to $20 to the patient vs PCR [polymerase chain reaction] multipanel respiratory virus panels that are $400 to $600 to the patient who hasn’t met their deductible. Interestingly enough, there aren’t huge differences in the sensitivity and specificity in them. A lot of these “throw-away tests” are very sensitive and specific, with accuracy in the mid-90% range. We’ve gone toward, what’s going to be most cost-effective to the patient and what’s going to give me an accurate diagnosis?

Peter Salgo, MD: If you do the rapid test, as you point out, that sensitivity and specificity is in the 90% range.

Jason Gallagher, PharmD: Yes, they’re very high.

Peter Salgo, MD: The PCR is a little better, but just by a few percentage points. Is that a distinction without a difference, or is it worth getting the PCR?

Jason Gallagher, PharmD: It depends on the situation; there’s our famous medicine or ID [infectious disease] answer. But another difference between these two is speed. If you’re talking about testing in an urgent care center, physician’s office, or a pharmacy that can use one of these rapid antigen-based tests, that’s a plus, even if they’re a little less sensitive or specific.

The other part that’s important to consider is that there are different situations when you use this test. There’s a patient who’s symptomatic, there’s a patient who’s asymptomatic, and so forth. The antigen tests are largely for symptomatic patients because their sensitivity is much lower if they’re asymptomatic. As we move toward this flu season or purported flu season, we may not care much about testing for flu right now, but we’re going to care about it going forward. I mentioned before that 99% reduction in positive tests, but there was only a 20% reduction in testing. People were still looking, as is appropriate. One of the big unfortunate problems that we have with these 2 illnesses is the incredible overlap in their symptoms.

Peter Salgo, MD: They’re both respiratory diseases, at least in large part.

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Transcript edited for clarity.

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