The Different Types of Tests

Tests can be broken down into two basic categories: Diagnostic and Serology (Antibody)

Diagnostic tests can be used to determine if you have an active coronavirus infection. These tests can be used by your healthcare provider or governing bodies to determine the next steps you should take regarding quarantine or isolation from others. There are two types of diagnostic tests for the detection of SARS-CoV-2: molecular and antigen.

Molecular tests detect viral genetic material which allow the virus to produce proteins and replicate during infection. The most common of these tests are polymerase chain reaction (PCR) tests. Generally speaking molecular tests are the most sensitive and accurate.

Antigen tests  detect specific viral proteins known as antigens. Antigen tests tend to be highly specific but are typically less sensitive than molecular diagnostic tests
*Antigens are the proteins that our bodies use to detect pathogenic material and mount an immune response and produce antibodies. The antibodies produced are specific to the antigen(s) detected and will specifically bind to the antigens to prevent an interaction of the antigen with healthy cells.*

Serology tests detect antibodies produced from the immune response that bind specifically to SARS-CoV-2 antigens. While diagnostic tests detect the presence of virus, serology tests detect the body's response to the virus. This means that serology/antibody tests should not be used for diagnostic purposes.
*We do not know how long antibodies stay in the body following infection with the virus that causes COVID-19. We do not know if antibodies give you protective immunity against the virus, so results from a serology test should not be used to find out if you have immunity from the virus. The FDA cautions patients against using the results from any serology test as an indication that they can stop taking steps to protect themselves and others, such as stopping social distancing or discontinuing wearing masks.*

Not All Tests Are Created Equal

is  A major factor in the varying uses and efficacy for determination of COVID-19 or SARS-CoV-2 infection is that each test is based on the detection of very specific targets associated with viral presence. 

Vertical Divider

Time to result

• ​potential for same day
• days - weeks (depends on location)

​Result indication

• Presence of viral genetic material in patient
• Is indicative of active coronavirus infection

Limitations

• Can't be used to indicate past infection

​
Follow up

• Molecular tests are typically highly accurate and do not need to be repeated or reflex tested

Vertical Divider

Time to result

• ​Same day 
• May be as fast as 15 - 30 min 

​
​Result indication

• Presence of viral antigenic protein in patient
• Is indicative of active coronavirus infection

​Limitations​

• Can't be used to indicate past infection
• More likely to miss an active infection by SARS-CoV-2 than molecular tests

​Follow up

• When symptoms are present but antigen tests are negative confirmation may be required by molecular testing.

Vertical Divider

​Time to results

• Potential for same day
• 1 -3 days is typical

Result Indication

• Presence of antibodies specific to SARS-CoV-2
• Is indicative of past coronavirus infection

​
​Limitations

• Cannot indicate current infection status whether positive or negative
• Cannot be used for diagnostic purposes

​Follow up

• May need to be followed up with second test for confirmation of antibody status
• May be followed up with molecular or antigen test to determine current status of active infection

Determination of Test Performance

No test is 100% accurate. It should also be noted that the determined performance of any test will vary based on disease prevalence. The performance of a diagnostic test can also be affected by the stage of disease. It is generally accepted that individuals who are very early or very late in the course of disease are likely to shed less virus than they would in the peak of infection. 

A test's performance is based on its sensitivity and specificity. Sensitivity can be though of as the ability of a test to correctly identify patients with a disease, while Specificity is the ability of a test to correctly identify people without the disease.

• A highly sensitive test will generally have a low false negative rate but will run a risk of false positives if the test’s specificity is low.
• A highly specific test will generally have a low false positive rate but will run a risk of false negatives if the test’s sensitivity is low.

Image source: Sensitivity vs Specificity, K. Steward

FDA Coronavirus Disease Testing Basics Document

Sources

1. Coronavirus Disease 2019 Testing Basics (FDA)
2. 3 Types of COVID-19 Tests, and Which One is the Most Accurate (A. Duncan, Health)
3. UC Davis: Different Types of COVID-19 Tests Explained
4. A Closer Look at COVID-19 Diagnostic Testing (FDA)
5. Sensitivity vs Specificity (K. Steward, Technology Networks)
6. GAO Science and Tech Spotlight: COVID-19 Testing