There are various tests to detect whether you have or were infected with the virus which causes Covid-19, SARS-Cov-2. However the kind of test you get, why you are getting it and when you get it is very important.
First of all, there are 2 basic types of tests:
- Blood tests. These measure antibodies.
- Swabs. These measure the presence of the actual virus or its genetic material. There are different areas that are used to measure these things.
- Nasopharyngeal (back of the nose) – the gold standard.
- Nasal cavity – second best.
- Oral/saliva swab.
- Sputum – lung secretions.
- Rectal/stool. YES you can measure viral particles/genetic material here as well.
What are they actually measuring?
- ANTIBODY tests. This is a blood test which measures whether or not you have antibodies to the virus. Same as an HIV or hepatitis C test. It can take a few days to get a result because it is measured in a lab. When exposed to something foreign, our immune system recognizes it as foreign and creates antibodies to various parts of foreign invader. There are 5 kinds of antibodies but when it comes to infection, the two basic ones are IgM which are called early phase antibodies, form very early in an infection and then fade within a week or two. The second type is IgG which starts to become measurable a week or two after initial infection. IgM would be positive early, during the time you are infectious. By the time you form IgG antibodies, you are probably no longer infectious. Antibody tests basically detect whether or not you were infected in the past. You may start to develop IgM antibodies while asymptomatic but will not have IgG antibodies yet. Antibodies are what we are talking about when the “antibody cocktail” is mentioned. Antibodies from infected people are concentrated down and injected into someone who is sick. In doing so, you assist their immune system to recognize and combat the virus faster.
- ANTIGEN tests. This is a swab test, also known as the “rapid” test. It measures whether you have viral proteins in your nose and/or saliva. These can be done on site and they can take anywhere from 15 minutes to a few hours to get a result and indicate active infection. If positive, you ARE infectious at that time, symptomatic or not. After about 10-14 days, this test is much less likely to be positive since the virus is now dead in the nose.
- GENETIC tests. Also a swab test, this measures genetic material from the virus. In this case, RNA. It is done by identifying the genetic material, whole or in part and amplifying it (through a process called PCR or Polymerase Chain Reaction) so that it can be detected. It is very sensitive but will also remain positive even months after infection since, through the amplification process, it can measure miniscule amounts of genetic material.
Timing of tests is important, depending on why you want to do the test. If you want to know if you HAD the infection recently or in the last few months, the blood test is the way to go. If you want to know if you are infectious at this moment in time, symptomatic or not, the antigen test, or rapid test, is the one to do, The genetic test, also a swab is very accurate but by the time you get the result, you may no longer be infectious.
All tests have a false positive (indicating infection when there is none) and false negative (indicating no infection when you are actually infected). The antibody test is the least prone to error. You either have the antibodies or you don’t. However, if you had Covid months ago, the antibodies do fade over time so the test may be negative. The swab tests can have a “user error” in that if the swab is not done properly, an adequate sample may not be obtained. If done properly, the genetic tests are prone to being positive long after infection. The best swab test is the one that makes your eyes water! The swab is placed straight back to the back of the nose, 2-3 inches back.
The bottom line is that testing is the best way to determine if you are sick with covid. The antigen tests are most likely to tell you if you are actively infected and contagious. The genetic tests may be positive when you are still contagious but by the time you get the result, you may no longer be infectious. The antibody test, the most expensive, basically indicates if you had covid.
Keep in mind that the 48 hours before symptoms is when you are most infectious and that it can take 3-14 days to develop symptoms after exposure (called the incubation period). The general rule of thumb is, that if you have had a significant, concerning exposure, there is no point in testing before 5-7 days since there will not be enough viral load to be detectable.
SO
If you think you have Covid or had a significant exposure, the best course of action is to self-isolate and speak with your doctor about what to do and when to do it.
In the meantime, do all the things you know will keep everyone else safe.
Stay safe and be well.
AC