Does a positive SARS (Severe Acute Respiratory Syndrome) antigen test confirm COVID-19 (Coronavirus Disease 2019)?

Does a Positive SARS Antigen Test Mean Positive for COVID-19?

Yes, a positive SARS antigen test indicates a COVID-19 infection, with high specificity (approximately 100%) but moderate sensitivity (63%), meaning false positives are rare but false negatives can occur. 1

Understanding SARS Antigen Tests for COVID-19

Antigen tests detect specific viral proteins (antigens) from the SARS-CoV-2 virus, which causes COVID-19. These tests provide results much faster than molecular tests (within minutes rather than hours or days).

Test Performance Characteristics

• Specificity: Approximately 100% (95% CI: 100% to 100%) 1

  • This means false positives are extremely rare
  • A positive result is highly reliable for confirming infection

• Sensitivity: Approximately 63% (95% CI: 56% to 69%) in asymptomatic individuals 1

  • Sensitivity is higher in symptomatic individuals at 81% (95% CI: 78% to 84%) 1
  • Sensitivity is highest in the first week after symptom onset (80.9%) 2
  • Sensitivity decreases in the second week of symptoms (53.8%) 2

Clinical Implications

When interpreting a positive SARS antigen test:

• Positive test: Due to high specificity, a positive result strongly indicates current SARS-CoV-2 infection
• Negative test: Due to moderate sensitivity, a negative result does not definitively rule out infection 3

Factors Affecting Test Accuracy

Several factors influence the accuracy of antigen tests:

• Timing of testing: Sensitivity is highest in the first week of symptoms 1
• Viral load: Higher viral loads correlate with better test sensitivity 1
• Symptom status: Tests are more sensitive in symptomatic (73.0%) versus asymptomatic individuals (54.7%) 2
• Sample collection quality: Proper technique is essential for accurate results 1

Clinical Decision Making

For symptomatic individuals:

1. A positive antigen test can be considered diagnostic for COVID-19
2. A negative antigen test should be confirmed with a molecular test (PCR) if clinical suspicion remains high 3

For asymptomatic individuals:

1. A positive antigen test indicates likely COVID-19 infection
2. A negative test has lower negative predictive value and doesn't completely rule out infection 1

Common Pitfalls to Avoid

• Assuming a negative test means no infection: Due to moderate sensitivity, false negatives can occur, particularly in asymptomatic individuals or those tested late in their illness 3

• Ignoring pre-test probability: In high-prevalence settings, even negative tests should be interpreted with caution 1

• Using antigen tests for "test of cure": The IDSA recommends against using repeat negative tests to guide release from isolation 3

• Misinterpreting cross-reactivity: While SARS antigen tests are designed to be specific for SARS-CoV-2, some cross-reactivity with other coronaviruses can occur, though this is rare with modern tests 4

In summary, a positive SARS antigen test should be interpreted as a positive COVID-19 result, while negative results should be interpreted with caution, particularly in symptomatic individuals or those with known exposure to COVID-19.