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Antigen Testing Guidance & Updates

The Centers for Disease Control (CDC) has released interim guidance on rapid antigen testing for COVID-19. Below is an explanation and summary of this guidance focusing on the elements that impact long-term care providers.

Key Points to Note

Important Updates and Guidance from AHCA/NCAL

  • Diagnostic Testing: Testing performed on an individual who is symptomatic, suspect of having COVID-19 or has confirmed exposure to COVID-19.
  • Screening Testing: Testing to identify asymptomatic individuals or individuals without known exposure to COVID-19 so that steps can be taken to prevent further transmission. Testing of staff or resident in long-term care facilities is an example of screening testing.
  • Surveillance Testing: Public health testing of large groups or the population to
    understand the prevalence and spread.
  • Sensitivity: % of all people with COVID-19 who test positive
  • Specificity: % of all people without COVID-19 who test negative
  • Positive Predictive Value: % of people who test positive who actually have COVID-19.
  • Negative Predictive Value: % of people who test negative who actually do not have COVID-19.
  • Pretest probability: Likelihood of a patient having COVID-19, which is based on:
    • the proportion of people in a community with COVID-19 (i.e., prevalence),
      commonly assessed as either the test positive rate in the community or the 7-day rolling average per 100,000
    • the clinical presentation of the patient (i.e. signs and symptoms of COVID-19)
    • their exposure risks (e.g. living with people who contracted COVID-19)

CDC outlines three situations in which antigen testing is particularly useful:

  1. For a person with symptoms suggestive of COVID-19, particularly in early stages of COVID-19 when viral load is the highest.
  2. For a person with a known exposure to COVID-19 to diagnose if they contracted COVID19 (note a person may take up to 14 days to develop the infection after exposure).
  3. Screening of individuals in high-risk congregate settings (such as a nursing home) who maybe asymptomatic or in a pre-symptomatic phase, in which rapid results and repeat testing afforded by point of care antigen tests can identify an individual with COVID-19 to help limit spread.
  • Any antigen test device used must have an Emergency Use Agreement (EUA) from the FDA. The Quidel Sofia2 and the BD Veritor are currently the only two point of care antigen testing devices with an EUA from the FDA.
  • Any nursing home conducting screening or diagnostic testing must comply with Clinical Laboratory Improvement Amendments (CLIA) regulations, which includes:
    • Obtaining a CLIA certificate
    • Meet all CLIA requirements to perform the testing. This includes assuring
      professionals conducting the test meet all training requirements for the test
      equipment used (training links available at the end of the document).
  • The CDC states that the reliability of test results is impacted by proper specimen
    collection and handling. This could include:
  • Inadequate specimen collection
    • Inadequate quality assurance procedures
    • Delays from sample collection to testing
    • Biosafety measures and instructions for use not being followed
  • Providers should refer to the CDC’s guidance on Collecting, Handling, and Testing
    Clinical Specimens for COVID-19 and also follow the manufacturer’s instructions.
  • The sensitivity of a rapid antigen test is lower than a PCR test, which means the false negatives are about 10-15% higher with the antigen tests compared to PCR tests.
  • The sensitivity can vary based on the viral load in a patient. A patient tested after five days of symptoms may drop their viral shedding load below the detection limit of the test.
  • The specificity of a rapid antigen test is generally as high as a PCR test, which means that false positives are unlikely with both tests.
  • Positive and negative predictive values of all diagnostic tests vary depending on pretest probability of the patient being tested.
    • The positive predictive value (PPV) and negative predictive value (NPV) varies based on the pretest probability. For example, assuming the antigen test has a sensitivity of 85% and specificity of 99%. This table explains the predictive values for different pretest probability:
  • CDC recognizes that controlling an outbreak depends on the frequency of testing and receiving rapid results, so serial antigen testing which is more possible with antigen tests may improve the positive predictive value in low prior probability situations, but how much is not yet known.
  • Interpreting rapid antigen tests should be based on the positive and negative predictive value of the tests which, as noted above, varies depending on the prevalence of infection in the community as well as the person’s clinical signs, symptoms, and history.
  • When using a rapid antigen test for diagnostic purposes:
    • Positive test results do not need to be confirmed, unless clinical suspicion of a false positive is high.
    • Negative diagnostic test results are considered “presumptive negative” and should be confirmed with a PCR test when the pretest probability is relatively high (especially if the patient is symptomatic or has known exposure) because of the higher number of false positives (see table linked above).
      • Consider the length of time the patient has been experiencing symptoms. Remember that a patient tested after five days of symptoms may drop below the detection limit of the test and produce false negative result.
      • If PCR testing is not available, the CDC indicates that clinical discretion can be used in whether to recommend the patient isolate. However, they do not recommend using antigen tests to decide when to discontinue isolation. Instead, providers should follow a time based strategy.
  • When using rapid antigen testing for screening purposes:
    • Test results positive and negative are generally considered presumptive and may need to be confirmed with a PCR. The CDC outlines a few exceptions to this, including: Page 4 of 4 August 20, 2020
      • You do not need to confirm a positive test result when the pretest
        probability is high (especially if the patient is symptomatic or has known exposure). However, when pre-test probability is low, you may want to confirm with another tests, but the individual should still be isolated until the positive test result is confirmed.
      • It may not be necessary to confirm a negative test if the pretest probability is low (especially if the person is asymptomatic and has no known exposures) or is part of a cohort (of people) that will receive rapid results on a recurring (e.g. weekly) basis such as in nursing homes.
  • All CLIA laboratory sites have to report results of antigen tests (negative and positive) to local or state health departments under the CARES Act.
  • HHS guidance on reporting laboratory data under the CARES act specifies what data should be collected and reported.

1. Who will receive the testing platforms and U.S. Food and Drug Administration (FDA)-authorized antigen diagnostic tests?

Nursing homes will receive either a Quidel Sofia 2 Instrument or Becton, Dickinson and Company (BD) Veritor™ Plus System over the coming months along with the associated FDA-authorized antigen diagnostic tests. To be eligible, nursing homes must have a current CLIA Certificate of Waiver AND meet certain epidemiological criteria. The list of nursing homes is posted on the Centers for Medicare & Medicaid Services (CMS) COVID NHSN data page and will be updated as new shipments go out. The U.S. Department of Health and Human Services (HHS) will distribute the testing platforms and FDA-authorized antigen diagnostic tests to all nursing homes with a CLIA Certificate of Waiver over the next few months.

 

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To access the Antigen Testing Recommended Protocol, please click here.

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