Testing for #COVID19 is rapidly changing as new technologies are being approved. Curious about the difference between the nasal swab test and the blood test? Are the point-of-care tests done on site as good as the ones sent to a lab? Read on to learn more!
Yesterday, I posted a thread describing the point-of-care (POC) and lab-based tests that diagnose an active infection by extracting the RNA from the viral particles and detecting specific sequences that are unique to COVID-19.
That drew a number of great questions about the “limit of detection” of these tests and the difference between nasal swab and blood tests, so I thought a new thread was in order.
Let’s tackle limit of detection (LOD) first. Why does this matter? An infection starts with a few virus particles which replicate into more as the infection proceeds. So, the lower the limit of detection, the earlier the detection and the better chance of diagnosing “mild” cases.
The FDA emergency use authorization paperwork for the Abbott ID NOW test says that it has an LOD of 125 genome equivalents/mL. That means that the test will be positive if there is RNA from 125 virus particles in each mL of sample (1 mL = 1/5 teaspoon).
google.com/url?sa=t&rct=j…
google.com/url?sa=t&rct=j…
But that doesn’t necessarily allow us to directly compare to the lab-based RT-PCR test. The reason why has to do with how the samples are processed. A nasal swab doesn’t give you liquid that you can directly test. Instead, the contents of the swab are washed into a solution.
The number of virus particles on the swab is a set value. But, depending on the volume of the solution, the concentration of virus/mL will change. If you use 2 mL instead of 4 mL, the concentration will be approximately double.
This makes it difficult to directly compare tests, as the volumes used may be different. What we actually care about is the LOD in number of virus particles on the swab. Alternatively, comparison can be done by using multiple different tests on the same people.
Every virus and test is different, but as we wait for data on COVID-19, it is interesting to take a look at the data for influenza detection. In this paper, researchers used a variety of POC tests and compared them to the lab test (RT-PCR). sciencedirect.com/science/articl…
The result? The POC tests aren’t quite as sensitive – they miss up to 11% of the positive results that the RT-PCR test catches. However, this is still quite promising for a test that can be done quickly and on large scale. 
Okay, so now to talk about the nasal swab test vs serological (blood) test. Compared to the example above, where the POC and lab tests are looking at the same thing – viral RNA – the nasal and blood tests are looking for different things.
When you have an active infection, your body recognizes that and starts making antibodies to fight off the infection. These are proteins (made of the same stuff as the protein coat of the virus) that bind to the virus and trigger your immune system to destroy it.
Just as the virus for each disease is unique, each antibody is unique. And, once your body finds the right antibody to fight the infection, it remembers how to make it so that it can fight that infection again later if needed. This is how immunity works.
Back to the tests. The nasal swab test is looking for the RNA from the virus, but the blood test is looking for the antibodies made by your immune system that bind to the virus.
As a result, the nasal swab test only detects an active infection – it will only be positive while you are sick and then go back to negative once your body clears all of the virus (or, more accurately, clears it to below the limit of detection).
In comparison, the blood test won’t be positive until you’ve had the infection for long enough to start making antibodies – usually several days. So, why is the blood test useful if you can’t detect the infection until later?
The blood test is useful in many ways. First, many people will have mild symptoms and never realize they were sick – widespread use of the blood test can reveal what percentage of the population has had the virus.
Second, evidence right now points to a lack of reinfection – that means once you’ve had COVID-19, you can’t get it again. Thus, the blood test can identify people who can safely be providing services and interacting with others without putting themselves at risk.
Lastly, while not an easily scalable treatment method, the antibodies from people who have had COVID-19 may be effective as a last resort treatment for people who are ill. The blood test can identify potential donors for such treatment. thelancet.com/journals/lanin…
So, the nasal or blood tests are not necessarily superior to one another – they are just different. Hopefully as our testing capacity increases, we will be able to start testing asymptomatic people and really understand the scope and patterns of the disease.
That’s all for now – thanks for joining! I’m not a doctor or epidemiologist, so I can’t say how this pandemic will all play out. But, as a chemist who does research with proteins and nucleic acids, I find the science fascinating and I hope you do also!
