A positive antigen test indicates that viral proteins from SARS-CoV-2, the virus that causes COVID-19, have been detected in a sample. Antigen tests are designed to be rapid, convenient, and relatively inexpensive point of care tests that can provide results within 15-30 minutes. However, there are important limitations to antigen testing that are critical to understand. A positive antigen test cannot definitively diagnose or confirm COVID-19 infection. Confirmation with a molecular test, such as PCR, is recommended.
How do antigen tests work?
Antigen tests detect specific viral proteins expressed on the outer surface of the SARS-CoV-2 virus. They work similarly to a pregnancy test, using antibodies that are designed to bind to these target viral proteins. When the viral proteins are present in a sample, they will bind to the antibodies on the test strip and generate a visual colored line to indicate a positive result.
Antigen tests have the advantages of speed, convenience, and low cost compared to molecular tests like PCR. Results are available in 15-30 minutes, no laboratory processing is required, and the disposable tests can be performed at the point of care. However, antigen tests also have lower sensitivity than molecular tests, meaning they are more likely to miss an active infection.
What are the limitations of antigen testing?
There are a few key limitations to be aware of with antigen testing:
Lower sensitivity than molecular tests
Numerous studies have shown that antigen tests have only 70-90% sensitivity compared to PCR tests, depending on the specific test that is used. This means 10-30% of active infections could be missed by antigen testing, leading to false negative results. The sensitivity is lower especially in the early and late phases of infection when viral load is low.
Positive results may need confirmation
Due to the potential for false positives, the CDC recommends confirming all positive antigen test results with a molecular test, such as PCR. False positives can occur if the test cross-reacts with a different pathogen or from user error in performing the test.
Does not detect prior infection
Antigen tests only detect active infection with live virus present. They cannot reliably determine if someone was previously infected with SARS-CoV-2. Antibody tests are needed to assess past infection.
Timing important for accuracy
Antigen tests are most accurate with symptomatic patients in the first 5-7 days following onset of symptoms when viral load is highest. Testing outside this window increases the risk of inaccurate results.
Variant detection uncertain
It is unclear if antigen tests maintain sensitivity and accuracy in detecting emerging SARS-CoV-2 variants. Ongoing evaluation is still needed.
When should antigen tests be used?
Despite their limitations, antigen tests can serve useful purposes when utilized correctly:
Rapid testing of symptomatic individuals
Antigen tests excel at quickly identifying infectious individuals with active viral replication. This allows for rapid isolation and contact tracing to prevent spread. Testing in the first 5-7 days of symptoms provides the most accurate results.
Frequent screening testing
Regular antigen testing such as every 2-3 days improves detection of infected individuals. This is useful in group settings like workplaces, schools, and congregate living facilities when performed repeatedly.
Confirmation with molecular testing
Positive antigen test results should always be followed up with molecular testing for confirmation before making definitive diagnoses or clinical decisions based on the result.
Situations where PCR not readily available
In locations without convenient access to PCR testing, antigen tests can assist with initial point of care screening and diagnosis integrated with clinical evaluation.
Can antigen tests detect Omicron and other variants?
Most FDA-authorized antigen tests were developed and evaluated using the original strain of SARS-CoV-2. However, based on the genetic sequences of emerging variants like Omicron, experts believe antigen tests should still detect the known variants but may have modestly reduced sensitivity.
Ongoing real-world data collection and laboratory evaluation indicate antigen tests, especially those utilizing multi-target detection, maintain reasonable sensitivity for Omicron but it is lower compared to prior variants. Some key points:
– Reduced sensitivity primarily occurs in asymptomatic individuals or those very early or late in infection.
– Performance in symptomatic individuals within 5-7 days of symptom onset is more preserved.
– Rapid serial testing improves overall detection of Omicron.
– Any patient with a high clinical suspicion for COVID-19 with a negative antigen test should receive follow-up PCR testing for confirmation.
The FDA monitors performance of authorized tests for emerging variants and issues updates on their website if certain tests show significantly reduced sensitivity that require modification. Leading health organizations currently still endorse proper use of antigen testing for screening and diagnosis integrated with clinical evaluation and confirmatory PCR testing.
How accurate are rapid at-home antigen tests?
Many rapid self-administered at-home antigen testing kits have been authorized by the FDA and Health Canada. These allow individuals to collect their own nasal swab sample and perform the test at home with results in 10-15 minutes. Convenience makes them useful for screening prior to gatherings or events.
Studies show at-home antigen tests maintain similar performance to tests performed by healthcare workers in terms of sensitivity and specificity. However, proper sample collection technique is important to achieve accurate results. Other key points about accuracy:
– 70-90% overall sensitivity compared to PCR remains the range for most authorized at-home antigen tests.
– Highest sensitivity in symptomatic individuals within the first 5-7 days after symptom onset.
– May miss asymptomatic or very early/mild infections with low viral load.
– Serial testing (every 2-3 days) improves detection rates.
– Positive results are presumptive and should be confirmed by PCR testing.
Despite slightly lower sensitivity compared to PCR tests, at-home antigen tests remain useful for rapid screening when used correctly at the right time during infection. Those at risk or with symptoms should follow up negative results with PCR testing.
How long after exposure can antigen tests detect infection?
It typically takes 3-5 days after exposure to SARS-CoV-2 for viral levels to rise high enough to be detectable on an antigen test. However, there is variability based on:
– Initial infectious dose received from the exposure
– Severity of symptoms
– Individual immune response
– Test sensitivity
On average, antigen tests are most likely to detect positivity around:
– Day 1 of symptoms: 40-50% sensitivity
– Day 2 of symptoms: 70% sensitivity
– Days 3-5 of symptoms: 80-90% sensitivity
So the chance of detecting infection rises steadily over the incubation period and peaks in the first 5 days of symptoms when viral load is highest.
Testing early after exposure when no symptoms are present yet leads to more false negatives. This is why repeat or serial testing improves detection of infection as viral levels rise.
How long does antigen test positivity persist?
Unlike PCR tests that can stay positive for weeks to months after initial infection, antigen test positivity begins to decline once an individual’s viral load starts decreasing and symptoms begin improving.
On average, antigen test positivity tends to persist:
– 10-14 days in mild to moderate cases
– 3 weeks or longer in severe or immunocompromised cases
However, there is significant individual variability. The timing depends on the patient’s immune response and how quickly their system suppresses viral replication.
In general, a positive antigen test means someone is still likely infectious to others. Those recovering from COVID-19 should isolate until symptoms have improved and two consecutive negative antigen results occur 24-48 hours apart.
Should I retest after a positive antigen test?
The CDC does not recommend retesting by antigen or PCR after a positive antigen test, even if the initial positive result is faint. Any positive line indicates detection of viral antigens. Retesting could remain positive for weeks and does not change guidance to isolate.
However, if a negative result occurs within 48 hours of the initial positive, repeat antigen testing may be warranted based on the clinical scenario. An indeterminate or invalid result may also warrant repeat testing.
It is important to note that retesting asymptomatically after isolation is not intended to establish clearance of infection or viability of the virus. This should be guided by symptom improvement.
How long to isolate after positive antigen test?
Those who test positive by antigen should immediately isolate for at least 5 days from the date of their positive result based on CDC guidelines. Isolation helps minimize spreading the virus when someone is infectious. The key considerations for ending isolation include:
– Must be fever-free for 24 hours without medication
– Other symptoms should be improving
– After the 5 day isolation, wear a mask for 5 additional days when around others to minimize risk
– If unable to properly mask, isolate for the full 10 days
– Continue monitoring for any return or worsening of COVID-19 symptoms
– Consider retesting after the 5 day isolation, and isolate again if positive
Properly timed antigen testing guides both initiation of isolation and assessment for lingering infectiousness before safely ending isolation. This allows for a data-driven, personalized isolation period.
How accurate is antigen testing for kids?
Most FDA authorized antigen tests have been evaluated in adults, but a growing number of clinical studies support their effectiveness in children. Key points about antigen testing accuracy in kids:
– Similar sensitivities 70-90% range compared to adults
– High agreement with PCR results, around 90% concordance in some pediatric studies
– Teenagers tend to have higher accuracy than younger children
– Proper sample collection technique is critical
– Serial or repeat testing improves detection, just like in adults
– More data needed in immunocompromised children
The FDA has granted special emergency use authorizations for certain antigen tests specifically in children, like the Abbott BinaxNOW and Lucira CHECKIT kit for home use. Parent-collected samples appear comparable to those collected by providers. Overall, antigen tests perform well in children and can assist with prompt identification and isolation of pediatric infections.
How should pregnant women use antigen tests?
Rapid antigen testing for SARS-CoV-2 provides important benefits for screening and diagnosis in pregnant women:
– Quick results enable early detection and management of COVID-19
– Avoids delays from relying only on PCR testing access
– Enables frequent screening through pregnancy
– Home tests facilitate safe social distancing
However, there are a few key considerations about use in pregnancy:
– Nasal swabs may need to be used over throat swabs due to gag reflex
– No data yet on accuracy with asymptomatic screening
– Make sure to do serial or repeat testing, and confirm positives with PCR
– Isolate with any symptoms and test for COVID-19
– Consult doctor with any questions or concerns about testing
When combined with other precautions like vaccination, antigen testing empowers pregnant women to take control of their health during the pandemic. At-home testing facilitates access and frequent monitoring through the course of pregnancy.
Can antigen test detect influenza or RSV?
The antigen tests that are widely available for SARS-CoV-2 specifically detect viral proteins unique to the novel coronavirus and are not designed to diagnose other respiratory viruses like influenza or RSV. Results will be negative with other viral infections, including:
– Influenza A and B
– Respiratory syncytial virus (RSV)
– Rhinovirus
– Adenovirus
– Common cold coronaviruses
– Parainfluenza
– Metapneumovirus
While antigen tests have excellent specificity for SARS-CoV-2, false positives are possible if a mutation significantly alters the target viral protein. But cross-reactivity with other circulating respiratory viruses is not a concern. A separate influenza rapid antigen test would be required to diagnose flu. PCR respiratory panels can detect multiple pathogens simultaneously.
Can I use an antigen test after COVID-19 vaccination?
COVID-19 vaccination does not interfere with antigen test performance and accuracy. Both unvaccinated and vaccinated individuals can use antigen tests as directed.
It is important to note the COVID-19 vaccines do not contain any live virus, so the viral proteins detected by antigen tests are not present after vaccination. Any positive antigen test in an asymptomatic individual after vaccination is due to actual SARS-CoV-2 infection.
Antigen tests remain useful for screening and diagnosis in vaccinated people:
– Breakthrough infections are still possible after vaccination
– No viable virus in vaccines that would confound results
– May need continued screening for work/travel based on regulations
– Helps differentiate COVID-19 from vaccine side effects
Post-vaccination, if symptoms develop that could be COVID-19, antigen testing is recommended to rule out active infection requiring isolation. Vaccinated people should continue other precautions like masking and antigen testing as appropriate based on their individual risk profile and exposures.
Do antigen tests detect the Delta variant?
The Delta variant (B.1.617.2) contains mutations in the spike protein compared to the original SARS-CoV-2 strain. However, experts conclude that available antigen tests accurately detect the Delta variant. This is supported by:
– Sequence analysis showing the antigen target regions are not significantly altered.
– Real-world data demonstrating continued reliable clinical performance.
– No evidence of excess false negatives suggestive of escape.
– Explicit confirmation by manufacturers after laboratory evaluations.
Some key points about antigen testing and the Delta variant:
– Sensitivity may be modestly decreased compared to the original strain but remains adequate.
– Serial testing improves detection, as with all COVID-19 variants.
– Any COVID-19 symptoms following exposure should prompt re-testing by PCR after a negative antigen test result.
– Those with possible Delta infections should isolate for 10 days minimum due to higher transmissibility.
The Delta variant remains detectable by properly utilized authorized antigen tests, especially in symptomatic patients. Enhanced precautions are still warranted in Delta predominant regions given the increased contagiousness of this strain.
Conclusion
Antigen testing is a useful rapid screening and diagnostic tool with unique advantages, but also limitations that require an understanding of proper use cases and cautions. While antigen tests have lower sensitivity than molecular PCR tests, especially early and late in infection, they can identify the most infectious individuals when utilized correctly. Those with COVID-19 exposure, symptoms, or a positive antigen test should isolate immediately until more definitive molecular testing and clinical evaluation can occur. Ongoing vigilance and research is still required to fully characterize the performance of antigen tests against emerging SARS-CoV-2 variants. Overall, antigen tests serve as an important complement to PCR testing and vaccinations in the ongoing public health response to COVID-19.