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Covid-19 Antigen Tests

Over a year into the Covid-19 pandemic, we’ve learned a lot—and we continue to learn more about this unique virus and how it spreads. Among other points, we’ve learned the importance of access to reliable and accurate testing in stopping the spread. Testing helps diagnose infection, with the goal of reducing asymptomatic (or presymptomatic) transmission and monitoring infection trends.

This blog will answer some common questions about Covid-19 testing, with a special focus on antigen testing.

What are the types of Covid-19 tests?

Covid-19 tests are categorized as either diagnostic tests (molecular, antigen tests) and screening tests (antibody tests). 

Diagnostic tests (like the antigen test) are appropriate for testing individuals who present symptoms consistent with COVID-19 (regardless of vaccination status) and/or have been notified of (or suspect) their potential exposure to a positive case. Timely and accurate diagnosis of COVID-19 is vital, so that proper isolation or quarantine measures can be followed to prevent the spread of Covid-19 to others. 

Screening tests (like the antibody test), on the other hand, look for signs of COVID-19 in individuals who may have no reason to suspect they’ve been infected. This includes asymptomatic individuals and those with no known exposure to any positive COVID-19 cases. This type of testing is important when you consider an asymptomatic, infected individual’s potential to spread the virus to others. With that in mind, screening tests are recommended for unvaccinated individuals, and often used in workplaces or school/university settings, before or after an individual travels, or for at-home testing for individuals who are asymptomatic with no known exposures to Covid-19. 

What is the difference between an antigen test and an antibody test?

Since their names are so similar, it’s worth pointing out the relationship between antigen testing and antibody testing. As previously discussed, antigen test results are primarily used for diagnostic purposes, while antibody test results serve more as a screening tool. 

Unlike antigen tests, antibody tests are not meant to detect current infection. Basically, antibodies are what the body’s immune system produces in response to antigens. When an antigen is present, it will have unique surface features that result in different responses from the immune system. In response, then, the immune system will create antibodies, proteins specifically designed to “fit” or lock onto the antigens. This binding process is what works to ultimately eliminate antigens from the body.

This is because antibody tests check for the presence of antibodies, which can take days or even weeks for one’s body to develop after initial infection—and because antibodies also may stay in someone’s blood for weeks after recovery is complete—viral tests are the more reliable test for identifying a current infection.

Diagnosing COVID-19 with PCR and antigen testing

There are two types of diagnostic tests available: nucleic acid amplification tests (NAATs, which include the PCR test) and antigen tests. 

What is the difference between COVID-19 antigen and PCR tests?

Both the PCR test and antigen test are classified as diagnostic tests, meaning they test for active infections of Covid-19. There are a couple key differences, though, which may help you better understand the options available to you (antigen test vs PCR). 

One difference is the turnaround time for results. You can expect results from a PCR test in as little as 24 hours, while antigen tests can produce results in just 15-30 minutes.

The other difference, then, is in what each test looks for as indicators of infection: PCR tests detect viral RNA, and antigen tests detect specific proteins on the surface of the coronavirus. 

What is a PCR test?

A PCR test works by detecting the presence of the virus’s genetic material using the reverse transcriptase polymerase chain reaction technique (or RT-PCR). If coronavirus RNA (define) is found, it is extracted from the sample and converted into DNA. That DNA is then “amplified,” which means many copies of the viral DNA are made in order to generate more measurable results.

While there are certain details that need to be accounted for to ensure maximal accuracy (e.g., proper sample collection, timing of testing within the course of infection, proper storage and handling of the sample), PCR tests are understood to be highly accurate, though it can sometimes take days to receive results. 

What is an antigen test?

Rather than looking for viral genetic material, antigen tests detect specific proteins on the surface of coronavirus molecules. 

These are commonly known as rapid tests, with results often coming within an hour of sample collection. Positive results are considered highly specific and therefore quite accurate, meaning a positive test is a reliable indicator of infection. Negative antigen test results, however, may need to be confirmed by a PCR test.

To further evaluate the differences between a PCR (one type of NAAT) and antigen tests, see Table 1 on the CDC’s website.

How is an antigen test done?  

An antigen test for Covid19 may be prescribed by a doctor, or can be performed without a prescription in certain testing scenarios, like being tested at a pharmacy or health clinic, or using an at-home test. In a majority of cases, antigen tests are “point-of-care” tests. This streamlines the antigen test procedure from sample collection to end result because the sample is collected and then analyzed in the same location, offering the benefit of quicker turnaround time—less time spent anxiously waiting for results.

There are no special instructions regarding preparation for antigen testing, other than letting the testing entity know about any recent COVID-19 symptoms. 

The test itself, then, takes less than a minute. The sample is collected through insertion of a cotton-tipped swab into one or both nostrils. The swab is rotated and then extracted. While it can be mildly uncomfortable, it is a very temporary discomfort.

How long does it take for results?

When administered as a point-of-care test, antigen test result time can be as little as 15 minutes. For at-home antigen tests, results take longer to receive due to the fact that the sample has to be sent off to be analyzed.

At-home antigen testing

The Food and Drug Administration (FDA) has authorized certain at-home antigen diagnostic tests capable of detecting active COVID-19 infections, regardless of the presence or absence of specific symptoms. These offer a quick and simple method for COVID-19 testing. There are mail-in kits available, wherein you can collect and ship your sample to a lab for analysis, as well as at-home test kits capable of producing results within fifteen minutes of sample collection. 

Currently, the FDA has authorized nine different home test kits from five different manufacturers for emergency use.

Understanding COVID-19 antigen test results

Correct interpretation of COVID-19 test results is important in determining next steps for the tested person. 

An antigen test positive means one or more specific viral antigens have been detected. Positive results can identify active infections so that the infected person can properly isolate themselves to prevent spreading the virus to others. On receipt of a positive test result, case interviews can be conducted in order to assess the patient’s close contacts who may also need to be notified and monitored. Generally, all positive-testing individuals, regardless of their vaccination status, should isolate themselves at home and remain in isolation until meeting the criteria for ending self-isolation precautions

An antigen test negative means no current signs of infection were detected at the time the sample is collected. Due to the “snapshot in time” nature of these tests, an individual’s test result may change if the same test is performed one or more days later—especially if there is known exposure to the virus. When a person tests negative and has no known symptoms or exposure, they are advised to continue following all recommended precautions for reducing transmission. These precautions, of course, include wearing masks, practicing social distancing, avoiding crowds, and so on.

Can false positives occur with antigen tests?

Although PCR is considered the “gold standard” of Covid testing, antigen test accuracy is typically understood to be rather high. They are not 100% accurate, however, due to a handful of variables that may be responsible for inaccurate results, such as incorrect sample collection or contamination of a collected sample. 

The possibility of a false-positive result is the highest in areas with low levels of community transmission, where the individual has limited potential contact with the virus. When the prevalence of the virus in a community is low and it is unlikely that an individual has been exposed to SARS-CoV-2, there is an increased risk of a false positive result.

Because antigen tests can be performed rapidly and at relatively low cost, they may be used in large screening programs that involve repeatedly testing people to help prevent the spread of SARS-CoV-2. Repeat testing helps mitigate the risk of false positives.

Can false negatives occur with antigen tests?

Due to the fact that antigen tests capture a moment in time, false negatives are another possibility. A person may test negative, despite infection, if they are tested either too soon or too late post-infection. The possibility of a false negative result arises from the nature of antigens. When an individual has COVID-19 but their viral load is low (like it may be when pre-symptomatic, in the early stages of infection, or as the virus is being cleared from the body), the risk of a false negative result arises.

How accurate is a covid19 antigen test?

In order to provide clarity, the CDC has specific guidelines regarding confirmatory testing based on antigen test results. A molecular test (like PCR) is commonly utilized to confirm antigen test results. 

If an individual tests positive despite having no symptoms, reporting no close contact with anyone with COVID-19, or living in an area with limited positive cases, a PCR test will be performed to confirm their status. A PCR test may similarly be recommended when an individual tests negative despite what seem like clear COVID-19 symptoms.

Should I get tested for Covid-19?

If you are concerned that you may be experiencing Covid-19 symptoms, you should schedule a viral test. You may also want to check out the CDC’s symptom checker tool.

When choosing which test to use, it is important to understand the purpose of the testing (diagnostic or screening), performance of the test within the context of the level of community transmission, need for rapid results, and other considerations. 

Should I get an antigen test?

While PCR tests are considered the more accurate diagnostic tool, an antigen test for COVID-19 would be appropriate when PCR tests are not readily available. In this type of case, a positive test result can be used to confirm infection in those experiencing symptoms indicative of COVID-19. 

An antigen test may also be recommended as a screening tool when an individual has been in close contact with a positive case, attended one or more large events or places of high transmission risk, live or work where many people live together (e.g., nursing home, correctional facility), or needs a negative test result in order to return to work or engage in other activities.

BioCollections has been at the forefront of testing for 20 years

Although the coronavirus has been with us now for over a year, BioCollections has over 20 years experience at the forefront of developing and delivering quick, reliable testing solutions at any scale. 

We have testing locations throughout the United States as well as internationally, standing by to provide quick and convenient COVID-19 testing to get you the peace of mind you need. Our employer testing solutions provide turnkey support including offsite or onsite visits, texted notification of results, and HIPAA-compliant tools to view results. 
Visit our website to learn more about our testing solutions, or you can contact us with questions.

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