Posted: Aug 8, 2020 / 07:43 AM EDT / Updated: Aug 8, 2020 / 07:43 AM EDT

WASHINGTON (AP) Ohio Gov. Mike DeWines positive-then-negative test results for the coronavirus are a reminder that no test is definitive.

The governor tested positive using a rapid test Thursday, before testing negative later in the day using a more sensitive laboratory-developed test. He was tested because he was going to meet with President Donald Trump during his visit to the state.

No test for coronavirus infection is perfect, and test results can be affected by a variety of factors, including the type of test used, the quality of the sample and when it was taken during the course of any infection.

A look at the types of tests currently used to diagnose COVID-19:

ANTIGEN TEST

DeWine initially received an antigen test, a type of rapid test that uses similar technology for screening for flu, strep throat and other infections on small machines at doctors offices.

Instead of detecting the virus itself, such tests look for proteins, or antigens, found on the surface of the virus, which are generally considered a less accurate measure of infection. The test, which uses a nasal swab, takes about 15 minutes.

The tests are relatively new in the U.S. and federal regulators have only allowed two on the market, from manufacturers Quidel and Becton Dickinson. DeWine spokesman Dan Tierney said a Quidel test was used for DeWine; a spokesman for Quidel said the company is looking into the matter.

The biggest risk with antigen tests is that they may deliver false negatives, missing real COVID-19 infections. But they are considered very accurate for ruling out the virus when it is not present, making false positive results highly unlikely.

Some experts have been calling for increased use of antigen tests because they can be developed quickly, avoiding the delayed results frequently seen with higher-grade tests that must be shipped to a laboratory for processing.

GENETIC TEST

After testing positive with the antigen test, DeWine said he and his wife received a genetic, laboratory-developed test that came back negative. This is the most commonly used test in the U.S. and is considered the gold standard for accuracy.

A nasal swab is mixed with chemicals to isolate any genetic material of coronavirus. Laboratory equipment is used to amplify the material millions of times, until it is detectable with a computer.

These tests take several hours to process and are done at large laboratories, hospitals or universities. They are considered the best method for both detecting whether the virus is present and ruling it out if theres no infection.

A third type of test uses technology similar to the genetic test but speeds up the process dramatically to deliver results in about 15 minutes. These tests done on small machines are used at the White House and at nursing homes to rapidly screen for the virus. But federal regulators consider them slightly less sensitive than the conventional laboratory test.

In either case, accuracy can be affected by a faulty nasal swab and where the person is in terms of the course of the infection. Even the most accurate genetic test usually only begins detecting virus three to five days after initial infection, when levels are high enough to be picked up.

Farnoush Amiri contributed to this report from Columbus, Ohio.

The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institutes Department of Science Education. The AP is solely responsible for all content.

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