Does a negative COVID-19 (Coronavirus Disease 2019) antigen test result indicate that an individual is no longer contagious?

A Negative COVID-19 Antigen Test Does Not Definitively Indicate You Are No Longer Contagious

A negative COVID-19 antigen test result alone does not reliably confirm that you are no longer contagious, as these tests cannot definitively rule out ongoing infectiousness. 1

Understanding Antigen Tests and Infectiousness

Antigen tests detect viral proteins and have important limitations when used to determine contagiousness:

• Antigen tests are less sensitive than molecular tests (NAATs), particularly after 5 days of symptoms when sensitivity drops to approximately 54% 1
• The Infectious Diseases Society of America (IDSA) panel found no empirical evidence supporting the use of antigen test results to determine infectiousness or to guide release from isolation 1
• A negative antigen test does not exclude the possibility of being contagious, though a positive result makes infectiousness more likely 1

Factors Affecting Test Results and Contagiousness

Several factors influence both test results and actual contagiousness:

• Timing of testing: Antigen tests are most sensitive early in illness (within 5 days of symptom onset) 1
• Viral load variation: The amount of virus can vary between the upper and lower respiratory tract
• Test limitations: Antigen tests detect viral proteins but cannot directly measure viable, infectious virus 1
• Individual factors: Data from Alaska showed that after 5-9 days of isolation, 54.3% of people still tested positive on antigen tests, with positivity more likely in:
  • Those with symptomatic infections
  • Those tested earlier (day 5 vs. day 9)
  • Those without previous infection or vaccination 2

Clinical Decision Algorithm for Determining End of Contagiousness

1. Primary approach: Follow time-based isolation guidelines rather than relying solely on test results

   • The IDSA specifically recommends against routinely repeating tests to guide release from isolation 1

2. If testing is used, consider:

   • A positive antigen test suggests potential ongoing contagiousness
   • A negative antigen test does NOT definitively rule out contagiousness 1
   • Multiple negative tests over consecutive days increase confidence but still don't guarantee non-infectiousness

3. Additional factors to consider:

   • Symptom status (resolution of symptoms, particularly fever for ≥24 hours without medication)
   • Time since symptom onset (longer duration reduces likelihood of contagiousness)
   • Immune status (immunocompromised individuals may remain contagious longer)
   • Exposure context (high-risk settings like healthcare facilities require more caution)

Common Pitfalls to Avoid

• False reassurance: Relying solely on a negative antigen test to determine non-contagiousness 1
• Ignoring symptoms: Continuing to have symptoms despite negative test results may indicate ongoing infection 3
• Single testing: A single negative test is less reliable than sequential negative tests
• Improper specimen collection: Poor technique can lead to false negative results
• Misinterpreting test purpose: Antigen tests were primarily designed for diagnosis, not for determining end of contagiousness 1

In conclusion, while antigen tests are valuable diagnostic tools, they should not be the sole determinant of whether someone is no longer contagious. Time-based isolation strategies remain the more reliable approach to preventing transmission.