Making Sense of COVID 19 TestingMay 28, 2020 2:29 pm
Making Sense of COVID 19 Testing
You hear that everyone should get tested for Covid 19, and there are so many different testing sites – each offering different things. Central Jersey Urgent Care and My InstaDoc Urgent Care alone have done 15,493 tests over the past two months. However, all these offerings can get confusing. Nasal swab test, spit test, antibody test, and now antigen test? We’re going to try to simplify all of this by walking you through all these tests.
There are three questions that testing can provide an answer to: Am I infected? Am I no longer contagious and can I hug my kids and go back to work? Was I infected in the past and didn’t even know I was?
Each post in our series will address each of these questions.
Am I infected with Covid 19?
There are two ways to test for the virus – the actual virus in your nasal passages and airways or our immune system’s response to the virus. To test if you are infected, we have to look for the actual virus, not the immune response.
Test type 1 – PCR. Say you get infected with Covid 19. You may or may not develop symptoms. Regardless, you’re still contagious. This is why we test people without symptoms. Even if you don’t develop symptoms, the virus is still hijacking the cells in your nasal passages and lungs to make copies of itself. As it does, it makes copies of its genetic code, RNA.
When you show up at one of our Covid testing locations, we would administer a nasal swab that we’ve all come to love to look for this RNA through a test called PCR. PCR takes a tiny bit of the RNA and makes enough copies of it in a machine so that it can be detectable. That is why it is a very sensitive test. It can pick up infections even when you have no symptoms.
But it’s not foolproof, for three reasons. The first reason could be because the collection wasn’t good. The swabs have to go back all the way to the nasopharynx, the back of your nose, to cover all the areas where the virus might be growing. If the collection is not good, you can get a negative result even if you are infected! That would be horrible – imagine you just kissed your grandma, reassured by the negative test! That is why we don’t allow patients to test themselves.
The second reason is the PCR machine. If you remember, the PCR machine makes copies of the genetic material until it can be detected. If the time on the machine is too short before calling it quits, you get a negative result even though there was a small amount of virus genetic material. That is why we don’t choose a rapid PCR machine.
The third reason is that you got tested too early and there was a very small amount of virus in your nose to start with. We can’t help that situation other than just watching for signs of infection and crossing our fingers.
That was the only broadly available way of testing for an actual infection until recently!
Test type 2 – Antigen Test. We are proud to say we now have the ability to administer an antigen test that looks for the shell of the virus as it makes itself home in your nasal passages. But this test is not as sensitive as the PCR nasal swab, so it is not as good as picking up infection in people who don’t have that many virus particles present in their nose yet. Then, why use it? Well, for one, it gives an answer in 15 minutes or less. Second, if it is positive, it is definitely positive, meaning that it didn’t detect some other coronavirus and called it positive for Covid 19 which some of the finger prick tests do. It is 100% specific for Covid 19. You come in with a fever, cough, diarrhea, headache, body aches, unexplained tiredness, shortness of breath, we would test you using this Covid 19 antigen test. If the Covid 19 test is positive, you’re isolating and definitely not hugging your kids.
Well, what if the Covid 19 antigen test is negative and you still have symptoms? Remember, that the antigen test is not as good as the PCR test in detecting the smallest amounts of virus. So, we would send a confirmatory test using the PCR. It’s sort of like getting a culture if the rapid strep test is negative or a urine culture if the urine test in the office is negative.
What if you don’t have any symptoms and you want to get tested for Covid 19? We would prefer to use the PCR nasal swab even though it is not as quick because the chances of a negative antigen test are very high. All negative antigen tests require a follow-up PCR test.
Am I no longer contagious? Can I hug my kids and go back to work to make a living?
The CDC recommends two ways to measure lack of contagiousness, symptom based and test based. If it has been more than 10 days since your positive test or the start of your symptoms and you have been symptom-free for 3 or more days (both qualifications have to be met), the CDC says you’re probably no longer contagious.
Most people take issue with “probably,” and as do many employers. So, the CDC also recommends a test based strategy where they require two negative PCR tests separated by 24 hours to be qualified as not contagious. That can be a problem as well. If you remember, the PCR looks for the genetic material (RNA) of the virus. When all the virus particles are killed off by our immune system, the dead virus particles still have their genetic material in them. The PCR picks this up and calls it positive. So, you can no longer be contagious and still test positive. Sometimes the test can remain positive for 4-5 weeks after getting better.
So, what can we do to sort all this out? We rely on the information that our immune system gives us. We use the antibody test. About 7-8 days after getting infected, your immune system would start making antibodies; first IgM and then IgG. IgM is a quick-fix antibody created by the body that is used to control the infection. Soon after, the body starts making IgG to remember the infection so it can fight it later. The CDC acknowledges that when your immune system starts making IgG, you’re no longer infectious. At Central Jersey Urgent Care/My InstaDoc Urgent Care, we send out a blood test for IgG which comes back in 1-2 days. If it is positive, you’re no longer infectious.
There is one thing to watch out for. You want to make sure that the antibody test is very specific, meaning that it won’t mistake a previous infection as an infection with Covid19. It would also be really bad to think that you’re no longer contagious because antibodies are detected, when the antibodies that the test detected were for some other infection. So, for that reason, we don’t use the quick finger prick tests. They’re not very good, yet.
Was I infected with Covid 19 in the past and didn’t even know I was?
The immune system makes IgG after an infection after about 8-10 days, if you get a reliable antibody test for Covid 19 and it is positive, you have been infected with Covid19. You’re lucky to have been infected and not develop symptoms, but at the same time, you probably spread it to others, not knowing that you were positive. That is why social distancing is still important.
Does a positive antibody test mean that you’re immune? As of now, no one knows the definite answer to that question. It may be that the antibodies prevent you from getting reinfected for four months, six months, or even three years. Though we cannot guarantee that a positive antibody test means you’re immune and at no risk for any future infections, it is likely that antibodies will confer some protection.(CDC).
Covid-19 Testing Process
Categorised in: blog
This post was written by CJ Urgent Care of NJ