Elon Musk

Elon Musk Said He Tested Positive and Negative for Covid-19—Here's What You Need to Know About the Different Tests

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On Friday, Elon Musk said that in one day he took four Covid-19 tests — called rapid antigen tests — and received two negative and two positive results. He also tweeted that he took two PCR tests at separate labs and is waiting for the results.

Musk, who has been dismissive about Covid-19 and its severity, tweeted that something “extremely bogus” was going on.

The truth is, no test is 100% accurate all of the time. But Musk's experience still begs questions: What are antigen and PCR tests? How accurate are they? And who should take them?

Here's what you need to know.

Rapid antigen tests

How it works: Rapid antigen tests require a nasal sample that can be collected at home or by a healthcare provider. They work by identifying specific proteins that are found on the surface of the novel coronavirus when someone is infected with Covid-19.

Although these types of tests are cheap to manufacture (they can cost $25-$100 out of pocket) and can deliver test results in a matter of minutes, they are not as reliable as polymerase chain reaction, or PCR, tests (more on that below).

Accuracy: The "sensitivity" rate, or how effective a test is in identifying people with antigens, is between 84-98%. According to the Food and Drug Administration, "positive results from antigen tests are highly accurate, but there is a higher chance of false negatives." For this reason, it's recommended that people get both a rapid antigen test as well as a PCR test (within two days) to confirm on a molecular level whether someone has Covid-19 or not.

In particular, Musk said he took rapid antigen tests from BD, the global medical company, Becton, Dickson and Company. While Musk's very small sample of four tests clearly only got the result right 50% of the time, in BD clinical studies, the tests achieved an 84% sensitivity rate. Musk said he also followed up with a PCR test. (BD did not immediately respond to CNBC Make It's request for comment.)

Who should take them: Rapid antigen tests work best when someone is tested in the early stages of the virus, when they have a high viral load. These tests are useful for screening in congregate settings, such as nursing homes and shared housing. The problem is that as the illness progresses antigen levels can drop lower than what the test can detect. That means a negative antigen test result doesn't rule out a Covid-19 infection.

PCR tests

How it works: For a PCR test, a healthcare provider will stick a long swab up your nostril to collect a fluid sample from the part of your throat behind the nose. These tests are designed to look for genetic material of the virus that causes Covid-19. This test is typically free with health insurance (but out-of-pocket charges for one test can be from about $60-$300), and results can take from one day to up to a week to come back.

Accuracy: PCR tests have a sensitivity rate of 95%. False positive results from a PCR test are unlikely, according to the CDC. This is because the genetic material from the virus that causes Covid-19 cannot be confused with the genetic material from other viruses, according to MIT Medical.

Who should take them: PCR tests are considered the "gold standard" test for a clinical diagnosis, according to the Centers for Disease Control. That means this is the type of test the average person should get. However, if you need results quickly, a rapid test along with a follow-up PCR test would suffice.

You should get tested for Covid-19 if you have symptoms of Covid-19, have had close contact with someone infected with Covid-19 or you've been referred for testing by a healthcare provider or public health official.

At the start of the pandemic, Musk tweeted a prediction that there would be “close to zero news cases” in the United States by April. The U.S. set a new high for seven-day average daily Covid-19 infections of 121,153 on Tuesday.

Check out: CDC now says wearing a mask protects the wearer, too

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