All information about Covid-19 keeps changing rapidly, as we learn more about it (3/25/21).
There are 2 broad types of tests for SARS-CoV-2 (the new coronavirus which causes Covid-19): tests to see if you have the virus, and antibody tests to see if you’d had it in the past. Mostly, we do the first type, to see if someone is infected. It’s done by inserting an uncomfortable swab to the back of the nose, although there is some evidence that the front of the nose, which is much less uncomfortable, can work as well. Saliva tests may be available in the future. Home tests are beginning to become available, with the advantage of not needing expensive protective equipment. For hospitalized patients, specimens may be collected from the lungs.
The main test to detect virus is performed by Polymerase Chain Reaction (PCR), which is so sensitive it can find the tiniest bits of virus. Other methods use antigen tests, which require more virus to turn positive, but may be just as useful. Positive results are almost always accurate, but negative results may be false-negative, meaning that the person really does have the virus. This can occur because testing was done too early in the disease, the specimen collection wasn’t good enough, or simply because tests are not 100% accurate. False-negative PCR tests occur about 15% of the time. If a person has very suggestive symptoms but the test is negative, it should be repeated in 1-2 days (not less than 24 hrs.). Antigen tests are more likely to be false-negative.
For a person with Covid, a negative test after 7-10 days or more from the beginning of illness means they are no longer contagious. A positive test, however, does not necessarily mean the person is still contagious, because PCR can detect dead viral particles which can’t spread disease. In technical terms, results should be reported in terms of a “Cycle Threshold” (“CT” value). The higher the CT, the more times they had to run the test to find virus, meaning the fewer viral particles around. CT values >34 may mean that the very few particles weren’t living; low CT values mean lots of virus. Unfortunately, labs usually don’t report this number (to the consternation of Clinicians).
If a person who has Covid then tests negative once, they should not get any more tests (unless a few months go by and they get new symptoms). Studies have shown that when you get over Covid, and a follow-up test is negative, a subsequent positive has simply found dead viral particles, which are meaningless. (However, some Covid mutations [variants] may enable active virus to remain longer; the CDC has not formally commented on this).
A big problem with Covid tests is that there is sometimes a long wait for results. Some tests give results in 15 minutes, others in hours. Some tests have had their accuracy studied more than others. But the bureaucratic process of some areas can cause delays of 1-2 weeks, due to staffing or whatever (it may be cheaper for some labs to perform the tests once a week, but they don’t tell you this). Covid is so new that there is no standardization to the way tests get done.
When should you get a Covid test? As soon as possible if you have symptoms like cough, fever, muscle aches, new loss of ability to smell. If a person has no symptoms but was exposed to someone with Covid, they could get tested a few days later. Being exposed is defined as having been less than six feet apart, inside without masks, for 15 min. in total. A person might also get tested if they were in a place for while where other people wound up testing positive. Nobody should ever get a Covid test “just to find out,” since negative today says nothing about tomorrow.
Antibody tests are not usually useful for individual patients, but they help epidemiologists track infection patterns. Antibody tests turn positive 2-3 weeks after illness, sometime remain positive, sometime don’t. The type of antibody detected is simply an indication of past illness; it does not mean immunity or protection from reinfection. There are various false-positives and false-negatives, making them much less useful for individual persons. It’s uncertain how or whether antibody tests will help a person who once had and recovered Covid know when to get the vaccine.