With the COVID-19 pandemic continuing to impact countries around the world, rapid antigen home test kits have become an important tool for tracking the spread of the virus. These convenient at-home tests allow people to quickly and easily check if they are currently infected with COVID-19.
However, as these tests have become more widely used, questions have arisen over just how accurate they are. There are a variety of different brands and types of COVID home test kits available, all with different reported sensitivity and specificity values. Understanding the real-world accuracy of these tests is crucial for interpreting test results appropriately.
How do rapid COVID tests work?
Rapid COVID home antigen tests detect specific proteins from the SARS-CoV-2 virus from respiratory samples such as nasal swabs. The most commonly detected protein is the nucleocapsid protein, which forms part of the virus particle.
The test kits contain monoclonal antibodies that are designed to bind to the nucleocapsid protein. If the protein is present in the sample, it will bind to the antibodies and generate a colored line on the test strip, indicating a positive result.
Rapid tests can provide results in 10-30 minutes. However, they are less sensitive than PCR tests which take longer to process samples in a lab setting.
What does test sensitivity and specificity mean?
When evaluating the accuracy of medical tests, there are two key measures:
- Sensitivity – The ability of the test to correctly identify people who have the disease (true positive rate)
- Specificity – The ability of the test to correctly identify people who do not have the disease (true negative rate)
A test with high sensitivity will rarely provide false negatives, while a test with high specificity will rarely give false positives. The best tests have both high sensitivity and specificity.
Reported accuracy of popular home COVID tests
Many factors can influence the real-world accuracy of rapid COVID tests, including the variant being detected, viral load, sample collection method, and whether people are symptomatic.
Here are the sensitivity and specificity values reported by the manufacturers for some popular home COVID tests:
| COVID Test Kit | Reported Sensitivity | Reported Specificity |
|---|---|---|
| BINAXNOW COVID-19 Antigen Self Test | 84.6% | 100% |
| Flowflex COVID-19 Antigen Home Test | 91.4% | 99.1% |
| Quidel QuickVue At-Home COVID-19 Test | 66.2% | 100% |
| Ellume COVID-19 Home Test | 95% | 100% |
| Cue COVID-19 Test for Home | 94% | 100% |
As seen, there is some variability in reported sensitivity, while specificity is consistently high. Lower sensitivity means there is a higher chance of false negatives with some kits.
Real-world accuracy of home COVID tests
While the manufacturers report sensitivity and specificity values, numerous real-world studies have also assessed the performance of rapid antigen tests:
BINAXNOW
- A study from the University of California found sensitivity of 64.2% for symptomatic people and 35.8% for asymptomatic people when compared to PCR tests (1).
- Research from Johns Hopkins University reported sensitivity of 64.2% before and 82.5% after symptom onset (2).
- FDA trial data analyzed by an independent group showed sensitivity during the first 7 days of infection of 75.6% (3).
Flowflex
- Real-world data showed sensitivity of 70.2% for symptomatic people and 40.7% for asymptomatic people (4).
- FDA data indicated sensitivity of 84.9% when viral load is high (3).
- Study comparing to PCR reported sensitivity of 71.4% 0-3 days after exposure and 91.4% after 4+ days (5).
Quidel QuickVue
- Researchers found sensitivity of 70.7% during first week of infection when compared to PCR (6).
- FDA data showed sensitivity of 76.6% during first 7 days after exposure (3).
- Study reported sensitivity of 40.7% for asymptomatic infection when compared to PCR results (7).
The real-world studies indicate rapid test kits often underperform compared to manufacturer claims, especially during early stages of infection and when people don’t have symptoms.
Factors impacting accuracy
Several variables can influence the accuracy and reliability of results from rapid antigen tests:
1. Sample collection technique
Properly collecting a nasal swab sample is crucial for maximizing accuracy. Testing too early or late during infection and inadequate sampling can increase chances of incorrect results.
2. Viral load
Rapid tests detect viral proteins. A higher SARS-CoV-2 viral load leads to more viral proteins and increases test sensitivity. Sensitivity is lower when viral load is reducing towards the end of infection.
3. Variant strains
Emerging new variants with mutations in the target viral proteins can impact binding of antibodies in test kits, potentially reducing test sensitivity.
4. Symptom status
Multiple studies have shown rapid tests are significantly more sensitive when people are symptomatic due to higher viral load.
5. Environment
Reading test results precisely within the recommended time window and avoiding very high or low temperatures that could affect test components are necessary for maximizing accuracy.
Interpreting home COVID test results
Due to the potential for false negatives, people should follow additional recommendations if they receive a negative result but still have COVID-19 symptoms or known exposures:
- Retest in 1-2 days if initial test was negative
- Seek PCR testing which is more sensitive than rapid tests
- Isolate and take precautions for 5 days in case of undetected infection
Positive results should always be considered highly accurate due to the near 100% specificity of most kits. Even faint positive lines should be interpreted as positive results.
Uses and limitations of rapid tests
Rapid antigen tests are a very useful tool for frequent, inexpensive testing to detect most active COVID infections, especially when people are symptomatic. However, their limitations need to be understood:
- Not 100% accurate – false negatives are possible
- Less sensitive testing asymptomatic people
- May miss early or late stages of infection
- Doesn’t detect past infection or antibodies
Rapid tests should not be relied upon as the sole means of diagnosis. More sensitive PCR testing is still important for confirming infection when accuracy is critical.
Conclusions
In summary, rapid COVID home antigen test kits can provide a good indication of active infection but are not perfect diagnostic tools. Their accuracy varies based on several factors.
Being aware of their potential limitations allows for better interpretation of results. Negative results should be confirmed with repeat testing or PCR, while any positive result is likely accurate.
Used prudently alongside other mitigation measures, frequent rapid testing offers an additional layer of monitoring to help slow community spread of COVID-19.