A Review of Antigen Diagnostic Tests

Antigen Test.jpg

What is an Antigen Test?          

Antigen tests are another form of diagnostic (viral) test available for the detection of current COVID-19 infections. Antigen tests detect specific viral proteins located in the coating layer of the virus [1]. Antigen tests, like molecular tests, are used to detect active current infection among individuals.

How Many Different Antigen Tests Are Available and at What Cost?

There are currently 7 antigen diagnostic tests approved by the FDA via the Emergency Use Authorization, which allows for the immediate use in the country [2]. The cost of each test can vary depending on the manufacturer, with prices ranging from $5-$200 [3]. Abbott’s BinaxNOW currently being promoted as $5 per test.  Like molecular tests, there may be additional costs incurred from hospital, lab, or physician fees. However, these costs are not passed on to the patient and instead are covered through public health insurance such Medicaid and Medicare, as well as private health insurances. Federal law requires private insurers, Medicare, and Medicaid to cover the cost of a COVID-19 test without any cost to the patient through the Families First Coronavirus Response Act and the Coronavirus Aid, Relief, and Economic Security Act [4]. These acts also provide grants to the uninsured as a way to get tested for free. However, patients may be responsible to pay out of pocket fees if they choose to go to out-of-network providers.

How are Antigen Tests Performed?

The samples collected for analysis follow a similar procedure to that of a molecular test. Nasal and oral swabs are used to collect a sample to analyze for viral presence. However, antigen tests do not require large special equipment to analyze the test samples, making it an optimal choice as a point of care (POC) test [4]. Currently, all 7 antigen tests are currently approved by the FDA through the Emergency Authorization Act [2], meaning they can be performed and analyzed in any setting, so long as the staff providing these tests are trained. 

How Long Until Someone Gets Their Results?        

As a POC test, the antigen test can provide results in less than an hour on average, and can be performed and analyzed in a variety of settings both within and outside of a healthcare facility [4]. This allows for the convenience of rapid testing and results rather than waiting days or even weeks with a molecular test. Abbott’s test, BinaxNOW, offers their product at $5 per antigen test with results in as little as 15 minutes [5]. As a POC test, antigen tests often offer a much simpler and easier testing protocol compared to molecular testing. Antigen tests do not require large specialized equipment, and since most tests are analyzed at collection sites, storage and handling requirements are almost negligible. This makes it easier to train personnel to administer the test.  

What Do Test Results Mean? 

A diagnostic test can show if a patient or staff member has an active coronavirus infection and should take steps to quarantine or isolate themselves from others. The antigen test, such as the Sofia- 2 or Abbott test, detects specific proteins from the virus in order to determine whether one has an active infection.

How Accurate Are the Results? 

The accuracy of antigen tests varies depending on what product test is being utilized. Antigen tests usually offer a higher level of specificity, indicating they are able to accurately rule out patients without the disease (true negatives). Unfortunately, one of the main drawbacks of the antigen test is that they usually offer a lower level of sensitivity compared to molecular tests, indicating that they are not as accurate in detecting patients with the disease (true positives). This results in a greater number of false negatives compared to molecular tests [6]. Because of this, a negative test is considered to be presumptive and should be followed with a confirmatory molecular test when symptoms are present.  False negatives can also arise from improper sample collection and handling. However, this can be addressed by providing frequent training to personnel, competency assessments in performing and analyzing a test, and proper storage policies.  

When Should Someone Get Tested?

Timing is another crucial factor that can affect the accuracy of a test. Like molecular tests, it takes days for viral load to increase to levels that are detectable using these tests. Testing too early may provide a false negative if the viral load has not reached detectable levels. The most accurate results are demonstrated starting 5 days after onset of infection and begin to lose accuracy after 14 days [7]. Some tests even recommend their use after 7 days of infection such as the BinaxNOW [6].

Because of the greater potential of false negatives results with antigen tests, they should mainly be reserved for patients who are symptomatic. Asymptomatic patient who receive a negative antigen result should not utilize this a definitive result, and should still continue to isolate from of others if they believe that they been exposed. Similar to molecular tests, no antigen test results should replace social distancing guidelines. 

Latest Posts
Antigen Table.jpg

Prepared in collaboration with graduate students from Keck Graduate Institute (Claremont,CA): Srbuhi Poghosyan, Diego Salinas, Larisa Malak Stepanian, Michael Thompson

References

  1. https://www.fda.gov/consumers/consumer-updates/coronavirus-disease-2019-testing-basicsAccessed December 3, 2020.

  2. https://www.fda.gov/medical-devices/coronavirus-disease-2019-covid-19-emergency-use-authorizations-medical-devices/vitro-diagnostics-euas, Accessed December 3, 2020.

  3. Peterson-KFF Health System Tracker. “COVID-19 Test Prices and Payment Policy.” https://www.healthsystemtracker.org/brief/covid-19-test-prices-and-payment-policy/, Accessed December 3, 2020.

  4. https://www.fda.gov/health-professionals/closer-look-covid-19-diagnostic-testing, Accessed December 3, 2020.

  5. https://www.abbott.com/BinaxNOW-Test-NAVICA-App.html#/, Accessed December 3, 2020.

  6. https://www.cdph.ca.gov/Programs/CID/DCDC/Pages/COVID-19/CDPH-Guidance-on-the-Use-of-Antigen-Tests-for-Diagnosis-of-Acute-COVID-19.aspx, Accessed December 3, 2020.

  7. Kucirka, Lauren M., Stephen A. Lauer, Oliver Laeyendecker, Denali Boon, and Justin Lessler. “Variation in False-Negative Rate of Reverse Transcriptase Polymerase Chain Reaction–Based SARS-CoV-2 Tests by Time Since Exposure.” Annals of Internal Medicine. Accessed December 1, 2020. https://doi.org/10.7326/M20-1495