Federal and state officials agree that expanded testing for COVID-19 will help get America back to work safely. But everyone has questions. You need to know the difference in what tests are available and what they mean. There are many FDA approved tests and all fall into three basic categories.

One of the most accurate tests to diagnose active COVID-19 disease is a polymerase chain reaction, or PCR, test. The PCR test has a high accuracy rate. The test is a mucus swab sampling for evidence of genetic material specific for coronavirus. This material is detectable if a person is actively infected. It is a very reliable test that can detect coronavirus within a few days of exposure. But this accuracy does come with trade-offs.

The downside is it takes several laboratory steps in order to amplify specific genetic material sequences to diagnose an acute infection. This requires a more sophisticated laboratory, time to do the test and is a bit more costly. Its high level of specificity for coronavirus also comes with the trade-off in terms of sensitivity. In other words, the more time that passes between the time the patient has symptoms and is tested, the higher the risk of a false negative result. There are some rapid PCR tests available and studies suggest they may miss up to 10% of cases.

PCR tests also require a high-quality specimen because you need a sufficient amount of intact viral RNA on the swab. That’s the reason you’ve probably read about the aggressive nature of obtaining the specimen deep within the nose as viral load rates vary considerably from person to person. So, you need a good specimen. This is the test that has been the workhorse during the pandemic.

The antigen test is the latest and most promising screening test. Rather than looking for genetic material like the PCR test, an antigen test is looking for viral proteins in the secretions. Researchers don’t expect it to be quite as sensitive as a PCR test, but it can be performed rapidly, fairly cheaply and on a broader scale. It is the same technology your doctor’s office uses for rapid strep or flu tests.

Positive antigen results are highly accurate. In some clinical studies there were zero false positives. But there is a chance for higher false negative rates than with the PCR method. Therefore, sick patients may need a PCR test before confirming a negative diagnosis if they have symptoms of disease. That’s the reason you’ll be asked a few questions if you get tested. Then again, the false negative rate was pretty variable even in PCR testing studies when the timing is considered between exposure, symptom onset and sampling.

The bottom line is antigen testing is vital to mass testing and sick patients may need confirmatory PCR testing as any test whether it is PCR or antigen has limitations. Both screening tools must be taken into clinical context in terms of exposure risk, symptoms and timing. The FDA has only granted emergency approval of two antigen tests to date. One is Quidel’s Sofia 2 SARS Antigen FIA test. The most recent is BD’s Veritor system.

While the PCR and antigen tests are designed to detect active disease, there is also an antibody test. Antibody tests will not identify active infection. But it will identify previous infection.

Antibody tests are a good way to track COVID-19 spread through a population. The test is conducted on blood rather than a swab. It detects antibodies that generally emerge after an infection. Immune responses may not occur until the second week after an illness so it is not a diagnostic test for active disease.

The value of antibody testing is determining if there was exposure and recovery from COVID-19. Theoretically, the presence of antibodies to COVID-19 indicates a low likelihood of being infected again. The problem is no one knows the degree of that immunity or how long it will last.

The current feeling is even when the COVID-19 pandemic is no longer spreading that it will then become an endemic disease occupying a constant presence within the population like influenza. Tracking tests will therefore be a continual part of disease tracking.