October 2020
Update on Antigen Testing
Danielle Snyderman, MD, CMD
In July 2020, CMS announced its plan to ship rapid antigen testing supplies and kits to all US nursing homes. By September 30th, over 14,000 facilities received either the Quidel Sofia or Becton Dickinson machine. Though the accompanying guidance emphasizes the role of rapid antigen testing in symptomatic individuals, the accuracy for use in asymptomatic residents and staff is less clear. Though those of us who practice in post-acute and long term care welcome support to test and minimize risk of outbreaks, we remain aware that this test is one that likely cannot stand alone and may have implications on both resident care and staffing patterns due to its limitations in accuracy and sensitivity. This review may serve to guide consideration for best practices as we continue to learn the best ways to optimize the health and safety of residents in our facilities.
 
 Principles of Antigen Testing
  • May augment other testing, especially when PCR testing is limited or delayed. (results>48 hrs.)
  • There are limited data to guide the use of rapid antigen tests as screening tests for asymptomatic persons to detect or exclude COVID-19, or to determine whether a previously confirmed case is still infectious.(CDC 9/4/20)
  • Compared to PCR testing, Ag testing:
  • Lower sensitivity (84-97.6%). Implication: more false negatives.
  • Similar specificity (nearly 100%). Implication: false positives are rare.
  • It may be necessary to confirm the rapid antigen test result with a PCR test, especially if the result of the antigen test is inconsistent with the clinical picture.
  • When confirming an antigen test result with the PCR test, the time interval between collection of samples should be less than two days (and assumes that there have been no new exposures). If the collection of the PCR test is more than two days from the original Ag test or if there have been opportunities for new exposures, the PCR should not be considered a confirmatory test, but rather a separate test. 
  • Antigen levels collected 5-7 days after onset of symptoms may drop below the limit of detection. (CDC 9/4/20)
  • Greatest accuracy potential is when there is a high pretest probability (symptomatic patient or high community prevalence).
  • Unknown accuracy (increased risk of false positive or false negative) when antigen testing is used for asymptomatic screening of residents or staff.
 
Requirements for POC Ag Testing
  • Must have Pennsylvania laboratory permit.
  • Must have a Clinical Laboratory Improvement Amendments (CLIA) Certificate of Waiver.
  • All positive, negative, and inconclusive/indeterminant results must be reported within 24 hours to Pennsylvania’s electronic surveillance system (PA-NEDSS).
Antigen Testing Devices
Ø  Quidel Sofia 2 (instrument based)
Ø  Becton, Dickinson and Company (BD) Veritor™ Plus System (instrument based)
Ø  Abbott BinaxNOW (does not require instrumentation) – from HHS & DOD
  • Received EUA (emergency use authorization), i.e., NOT FDA cleared/approved
  • Nasal swab. Testing time: 15 minutes
  • Minimal testing was done in asymptomatic patients


Device: Quidel Sofia 2
Specificity: 100%
Sensitivity: 96.7% (if done within 5 days of sx)

Device: BD Veritor
Specificity: 100%
Sensitivity: 85% (BD disagrees & reports results similar to Sofia2)

Device: Abbott Binax
Specificity: 98.5%
Sensitivity: 97.1% (if done within 7 days of sx). 75% if tested >7d from sx

Device: All tests
Specificity: 84-97.6%
Sensitivity: Close to 100%
Interpretation of Results of Antigen Testing (CDC)
  • When antigen testing is POSITIVE:
  • If symptomatic: consider POSITIVE (new case).
  • If asymptomatic but + facility outbreak: consider POSITIVE (new case).
  • If asymptomatic and no outbreak: exclude from work, PCR < 48 hrs.

  • When antigen testing is NEGATIVE:
  • If symptomatic: exclude from work, check PCR.
  • If asymptomatic but + facility outbreak: continue serial facility testing.
  • If asymptomatic and no outbreak: continue serial facility testing.
 
 Suggested Use of Antigen Testing
  • Guidelines for use are still resolving and a consensus has yet to be reached on best practice.
  • PCR testing remains the gold standard of testing, but antigen testing may offer augmentation to onsite point of care decision making in an expedited fashion.
  • For symptomatic residents/staff: antigen testing may be useful as long as less than 5-7 days since onset of symptoms. If greater than 5-7 days, PCR testing should be used.
  • For asymptomatic resident/staff: Use PCR if turn around time is less than 48 hours but if >48 hours can consider antigen testing.
  • Avoid antigen testing if symptom onset is more than 5-7 and if no facility outbreak or low community rates.
 
References
Sources: CDC, 8/27/20, CDC, 9/4/20, PA-HAN 526, 9/17/20, CMS guidance on CLIA waiver PA-HAN 526, 9/17/20. Special thanks to Leon Kraybill, MD for generously sharing his many resources.
Any views or opinions presented in this article are solely those of the author and do not necessarily represent any policy or position of PAMED, PMDA, AMDA, its affiliates and members.
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