Many people want to be tested for the SARS-COV-2 virus, not because they are sick, but to make sure they are not infectious. This is important for people to know in certain work situations where they may be at higher risk of infection, to know if it is safe to visit their grandchildren, grandparents or any other person who might be vulnerable to severe disease from COVID-19. It also may be important for young people returning to in person education in school or college settings.

In this post  I will outline the tests that are available now as well as some that may be available soon. I will discuss how sensitive and specific each one is as well as how many days after infection a test is a reliable indicator of the presence of infection with COVID-19.

Incubation Period

Incubation period is the time from exposure to the virus to development of symptoms. This ranges from two to fourteen days, but the median (middle) is 4-5 days. Because of this variable incubation period, it is difficult to predict how soon a test for the virus will be positive. It could be as soon as one or two days after exposure, or as much as 7 days after exposure. People shed the most virus and are therefore most infectious 48 to 72 hours before they get symptoms.

What does a negative test for SARS-COV-2 mean?

A negative test for the virus could mean one of two things:

1. You are not infected with the virus
2. You are infected with the virus but are still in the incubation period, so your test is not yet positive.

If you know the date you were exposed, then a negative test 7 days later is good evidence that you are not infected. If you don’t know whether you have been exposed, then one negative test does not distinguish between 1 and 2. In order to be sure that you are not infected, you would have to have a second negative test a week later. If you are having symptoms of COVID-19 and have a negative test, that is good evidence that your symptoms are not due to COVID-19.

PCR Tests

PCR stands for polymerase chain reaction. These tests use a tiny amount of virus recovered from a swab and make millions of copies of part of the viral RNA, which  is then enough to measure. This is the most sensitive test for detecting SARS-COV-2. False negatives and false positives are rare (as long as you are not still in the incubation period).

The standard PCR test requires a nasopharyngeal swab. This means that a swab is put in the nose and pushed all the way to the top of the throat. Not only is this not very pleasant, but it has to be collected by someone wearing full PPE (personal protective equipment) because obtaining the swab can induce coughing and therefore has the potential to release coronavirus particles into the air. The testing is done by large machines at reference labs like Quest and Labcorp. These machines can run multiple samples at a time and give results in about an hour. The commercial labs have been overwhelmed by the huge number of swabs they have received from places like Arizona, Texas and Florida. Delay in obtaining test results is common and can be up to 10-14 days in some instances.

Point of Care PCR Tests

Abbot ID Now. It has received emergency approval from FDA. The machine is about the size of a toaster and is small enough to use in doctor’s offices and pharmacies. It works with nasal or throat swabs which can be obtained by the patient, and thus does not require a technician wearing full PPE. This is the machine that is used for testing president Trump and the people around him. Results are available in about 5 minutes for a positive test and about 13 minutes for a negative test. This is a PCR test.

Mesa Biotech Accula. It has also received emergency approval from FDA, This is a very small machine that fits in the palm of your hand. It is too complicated to use as an at-home test, but would be suitable for clinics, pharmacies and perhaps schools. Test results are available in about 30 minutes. This is also a PCR test.  This test uses just a nasal swab. This can be done by the person being tested, so does not require PPE.

Point of Care Antigen Tests

Antigen tests are used to test for the presence of proteins from SARS-COV-2. This is the same technology used for the rapid strep test and for rapid flu tests. These are rapid tests, with results in a few minutes. This is a much less sensitive method than tests that use PCR. Antigen tests miss up to 20% of infected people, that is, there are a high number of false negatives. False positive antigen tests are rare, so a positive result means a high likelihood of current infection. If you have symptoms that might be due to COVID-19 and an antigen test is negative, that negative should be confirmed with a PCR test.

You cannot safely use a negative antigen test as proof that you are not infected and are safe to interact with other people, especially vulnerable people who are older or who have underling conditions.

At Home Tests

The holy grail of COVID-19 testing would be a test that people could do at home. Lots of companies are working on such a test, which would be similar to a home pregnancy test. It should work with a simple nasal swab or a saliva sample. No such test has yet received emergency approval from the FDA, but stay tuned. It is likely that such a test will soon be available.

At present there are some tests that use saliva or a nasal swab collected at home, but the sample than has to be mailed to a commercial lab. That does not really count as a home test., although it does mean that no one wearing full PPE has to collect the sample. That is definitely a plus.

Antibody Tests

As I reported in a previous post, antibody tests are used to detect previous exposure to SARS-COV-2.  Unless the population being tested has had a high percentage of people infected, positives with this test are likely to be false positives. Antibody tests are therefore not of much use to individuals and are mostly used by epidemiologists to estimate the percentage of a population that has been infected with SARS-COV-2.

Bottom Line

Lets say you want to visit your grandchildren or your elderly parents and you want to be sure that you do not have COVID-19. In that case, you would need two negative PCR tests a week apart.

Antigen tests are much faster and cheaper. If you get a positive test, you are very likely infected. If you get a negative test, it does not tell you much.

Home tests are not yet available, but likely will be reasonably soon.