False Positive Rates for Rapid Flu and COVID-19 Tests
Rapid COVID-19 molecular tests (NAATs) have essentially zero false positives with a specificity of 100% (95% CI: 98-100%), while rapid COVID-19 antigen tests also demonstrate 100% specificity (95% CI: 100-100%), meaning false positive rates are negligible at 0-9 per 1000 tests across various prevalence settings. 1
COVID-19 Rapid Molecular Tests (NAATs)
Specificity and False Positive Performance:
- Rapid molecular NAATs demonstrate pooled specificity of 100% (95% CI: 98-100%) in symptomatic individuals 1
- In asymptomatic individuals with known exposure, rapid NAATs show specificity of 99% (95% CI: 95-100%) 1
- False positive results range from 5-10 per 1000 patients tested across different prevalence scenarios (5%, 20%, 50%) 1
- The IDSA guidelines emphasize that positive results from rapid NAATs do not usually need confirmation by standard laboratory-based tests due to their high specificity 1
Clinical Context:
- The extremely high specificity means false positives are rare enough that confirmatory testing is not routinely recommended for positive rapid molecular results 1
- This applies to both rapid RT-PCR and isothermal amplification tests (like Abbott IDNow) 1
COVID-19 Rapid Antigen Tests
Specificity and False Positive Performance:
- Antigen tests in symptomatic individuals show pooled specificity of 100% (95% CI: 100-100%) 1
- In asymptomatic individuals, antigen tests demonstrate specificity of 100% (95% CI: 100-100%) 1
- False positive results are 0 per 1000 tests across prevalence ranges of 1%, 5%, and 10% in asymptomatic populations 1
- In symptomatic populations, false positives range from 0-9 per 1000 tests depending on prevalence (5-50%) 1
- A systematic umbrella review found false positivity rates in rapid antigen tests range from 0.0% to 4.0% 2
Important Caveat:
- The primary limitation of antigen tests is lower sensitivity (63% in asymptomatic, 80-90% in symptomatic), not specificity 1
- False negatives are the concern with antigen testing, not false positives 1
Influenza Rapid Tests
Evidence Gap:
- The provided guidelines focus exclusively on COVID-19 testing and do not contain specific data on influenza rapid test false positive rates [1-3]
- General medical knowledge indicates influenza rapid antigen tests typically have specificities of 90-95%, translating to false positive rates of 5-10%
- Molecular influenza tests (PCR-based) have higher specificity similar to COVID-19 molecular tests
Clinical Implications and Pitfalls
Key Practice Points:
- False positives with COVID-19 rapid tests are extremely rare and should not be a primary concern when interpreting positive results 1
- The consequences of false positives include unnecessary isolation, anxiety, delayed investigation for true causes of symptoms, and inappropriate COVID-19 treatment 1
- However, given the near-100% specificity, these consequences occur in fewer than 1% of positive tests 1
Common Pitfalls to Avoid:
- Do not routinely confirm positive rapid molecular COVID-19 tests with laboratory-based NAATs—the specificity is high enough that this is unnecessary 1
- Do not dismiss positive antigen tests as false positives without strong clinical reasoning—their specificity is excellent 1
- Be aware that in very low prevalence settings (1%), even with 100% specificity, the positive predictive value decreases, though false positives remain rare 1
- One research study suggested theoretical false positive rates up to 80% in very low prevalence asymptomatic screening, but this was based on theoretical modeling and was subsequently withdrawn 4
When to Consider Repeat Testing:
- If clinical presentation is highly inconsistent with COVID-19 and an alternative diagnosis is more likely, consider the possibility of a false positive 5, 6
- In extremely low prevalence settings (community prevalence <1%), positive predictive value decreases even with high specificity 1
- Document exposure history and symptom timeline carefully, as these affect interpretation 3