What to Know About the COVID-19 Antibody Test

There has been a lot of news surrounding the use of antibody tests for COVID-19 as well as confusion about what the tests do. How are they different from PCR tests used for the primary diagnosis of COVID-19? In the most basic terms, antibody tests are used to determine if you have had a past COVID-19 infection, while PCR tests are used to determine if you are currently infected.

The COVID-19 antibody test is a type of serology test, or blood test. This immune response test detects the immune proteins—called antibodies—that the body produces in response to the virus. It does not detect the virus itself.

What Is an Antibody?

Whenever the immune system is confronted with a disease-causing organism like a virus, it produces defensive proteins called antibodies that are matched specifically to that organism. The antibody "recognizes" the invader by proteins on its surface called antigens. This allows the antibody to target the invader to either kill it directly or latch onto its antigens so that it can be "tagged" for neutralization by other immune cells.

COVID-19 antibody tests are looking for the antibodies made in response to exposure to the SARS-CoV-2 virus.

There are several different types of antibodies, which are also called immunoglobulin (Ig), that the body can produce in response to an infection. They include:

• Immunoglobulin M (IgM): The first antibody the immune system produces when confronted with a virus or other disease-causing pathogens. It accounts for roughly 10% of all antibodies that the body produces.
• Immunoglobulin G (IgG): This takes longer to produce than IgM, but is the predominant antibody found in blood and other body fluids. It also plays a role in creating "memory" cells (called memory B lymphocytes) that remain on guard after the infection has cleared, ready to attack should the invader return.

How the Tests Work

There are two different technologies used for COVID-19 antibody test. The first, called an enzyme-linked immunosorbent assay (ELISA), is a lab-based test that confirms the presence of an antibody by exposing it to its corresponding antigen. The second, called a lateral flow assay (LFA), follows the same principles of ELISA but is used for on-site rapid testing.

 The test itself can be performed in two different ways:

• Finger-stick capillary blood testing is used for rapid testing. This LFA-based test involves taking a small sample of blood from a finger prick and exposing it to a chemical reagent in a disposable device similar to a pregnancy test. Within minutes, the appearance of colored lines indicates whether antibodies are present or not.
• Venous blood testing is an ELISA-based test that requires a blood draw from a vein. After spinning (centrifuging) the blood to separate the serum from cells, the serum is diluted and added to a testing tray well that is coated with COVID-19 antigen. An enzyme tracer is then applied. If there are antibodies in the sample, the binding of the antibodies and antigens will trigger a color change. It can take 24 hours or longer to get results.

Based on changes in color or the appearance of colored lines, COVID-19 antibody tests can be interpreted in one of three ways:

• A positive result means that you have previously been exposed to COVID-19 as evidenced by the presence of IgG and/or IgM antibodies.
• A negative result means that you have either not been infected or you have been tested too early in the period of time between infection and antibody production. For COVID-19, this is estimated to be between one to three weeks.
•  An indeterminate or borderline result could mean that you tested too early or that an error occurred in the lab or during the blood collection or delivery. Whatever the cause, a repeat test would be indicated.

When to Take a Test

A COVID-19 antibody test is not used to diagnose an active (acute) infection. It is only used after the body has produced enough antibodies to reach detectable levels. For IgG antibodies, this usually takes 11 to 18 days from exposure. For IgM antibodies, detection may be achieved by as early as four to five days after exposure.

If you test too early, you may get a false-negative result. A false-negative means that you have been infected even if the test says otherwise.

Even though IgM antibodies are detectable early in an infection, they tend to dissipate quickly before being replaced by IgG as the predominant antibody. The amount of IgM produced is also highly variable from one person to the next, making it a less reliable marker for infection.

The window of opportunity for COVID-19 antibody testing is unknown. While IgG antibodies can persist at detectable levels for months and years, scientists aren't yet sure if this is the case with COVID-19. Part of the reason for the large-scale antibody surveillance is to find out.

Why COVID-19 Antibody Tests Are Used

The COVID-19 antibody test was designed primarily for population-based surveillance and research but may also have applications for individuals exposed to the virus.

Epidemiologic Research

For epidemiologists, widespread antibody testing can help detail the actual scale of the pandemic (including fatality rates and disease prevalence) and provide a clearer picture as to which populations are at greatest risk of severe illness and death. This information can be used by health officials to formulate a more tailored response to the disease if or when a subsequent outbreak occurs.

The Centers for Disease Control and Prevention (CDC) intends to conduct widespread antibody testing, focusing on hard-hit regions like Washington state and New York City, to help answer the following questions:

1. How much of the U.S. population is actually infected?
2. How many infected people experienced mild to no symptoms?
3. How has the pandemic changed over time?
4. What are the risk factors associated with infection, severe illness, and death?
5. How long do antibodies remain after infection?
6. How does the U.S. pandemic differ from what was seen in other countries?

These pieces of information can help scientists determine if COVID-19 is becoming more virulent and what the appropriate response should be if there is a rebound in new infections.

Individual Testing

From an individual standpoint, a COVID-19 antibody test may be less useful. Although a test can confirm if you have been infected (even if you never had any symptoms), it cannot tell when you became infected or indicate whether or not you have developed immunity to the virus. Although recovery from other coronaviruses, like SARS and MERS, typically confers to some level of immune protection, COVID-19 may not act in the same way.

One way that antibody testing can help on an individual level is by identifying candidates for an experimental therapy known as convalescent plasma exchange. The U.S. Food and Drug Administration (FDA), in partnership with the American Red Cross, is encouraging people 17 and over who weigh more than 110 pounds to donate blood if they have a positive COVID-19 antibody test result and are in good health. The transfusion of defensive antibodies may help severely ill people better fight and recover from a COVID-19 infection. More research is needed.

Testing Options

Due to the urgent need for diagnostic tests for COVID-19, the FDA issued an Emergency Use Authorization (EUA) order on February 29, 2020, allowing for the manufacture and distribution of COVID-19 tests without the need for a formal approvals process. In place of an FDA review, manufacturers have 10 days from the time of the release of their product to send validation of the test’s efficacy and safety, after which temporary authorization is granted.

While the initial authorizations were focused on PCR tests, the first COVID-19 antibodies test received EUA on April 1, 2020. Others have since been authorized.

Unauthorized Tests

Confusion over the FDA’s regulatory flexibility has led some companies to mistakenly claim that their tests, many of which are imported from China, are "FDA-approved." Other unscrupulous online resellers have taken to shipping fake tests directly to consumers.

According to a congressional subcommittee overseeing COVID-19 testing, the FDA currently has "no insights into how many (COVID-19 antibody) tests" are being distributed in the U.S., while asserting that many of the tests are of "frankly dubious quality."

To ensure that you get a EUA-authorized test, call the lab in advance and ask which antibody test it uses. New tests are being approved weekly, so you may need to call the FDA at 1-888-INFO-FDA (1-888-464-6332) for an updated list if you are unsure.

How Accurate Are the Tests?

To ensure the most accurate COVID-19 antibody tests possible, the FDA has established performance thresholds necessary for EUA authorization:

1. The test should have an estimated sensitivity and specificity of at least 90% and 95%, respectively. Sensitivity is the ability of a test to detect pathogens and correctly identify people with a disease. Specificity is the ability of a test to correctly differentiate pathogens and identify people without a disease.
2. The manufacturer must be in the advanced stages of efficacy testing and have demonstrated safety data. Even though specific tests are required for FDA authorization, the results need only be validated by the manufacturer.

False-negatives are more common with COVID-19 antibody tests, due in part to the variable sensitivities of the tests. Compared to venous blood tests, rapid finger-stick tests tend to be less reliable and more likely to return a false-negative result.

False-positive results are uncommon but can occur. Because COVID-19 belongs to a large family of coronaviruses, the test may inadvertently detect the antibody of related coronavirus strain (such as the HKU1, NL63, OC43, or 229E strains associated with the common cold) and trigger a false-positive reading.

It is important to remember that the sensitivity and specificity of the authorized tests are estimates only. In a real-world setting, the tests often fall short. According to research published in the journal MedRx, the real-world false-positivity rate of COVID-19 antibody tests ranges anywhere from 0% to 16%. The false-negativity rate also varies, rising to as high as 19%, particularly if the tests are performed prematurely.

What to Expect

The COVID-19 antibody testing procedure and turnaround times vary by the assay—or method of analysis—used.

Venous Testing

ELISA-based antibody tests are blood-based tests that require a blood draw from a doctor, nurse, or phlebotomist. The blood draw may cause temporary discomfort for some people, but it is usually not painful. The process is something most people are familiar with.

To perform the blood draw:

1. An elastic band or tourniquet is placed around your upper arm to make a vein swell.
2. The injection site, usually near the crook of the arm but sometimes nearer the wrist, is cleaned with an antiseptic swab.
3. A straight needle or butterfly needle is inserted into the vein.
4. Between 8 and 10 milliliters (mL) of blood is extracted into a vacuum-sealed test tube.
5. The needle is removed, and a small bandage is applied to the puncture site.
6. You will be asked to maintain pressure on the injection site for a few minutes to ensure there is no bleeding.

Side effects tend to be mild and may include injection site pain and bruising. Call your doctor if there is increased redness or pain, swelling, fever, or visible discharge. Infection is rare but may occur.

Once the blood sample is sent to the lab, the turnaround time can be anywhere from one day to several days. ELISA tests are fully automated and can usually be completed in the lab within 90 minutes.

Fasting is not required for the COVID-19 antibody test.

Rapid Testing

LFA-based rapid testing can be performed by a doctor, nurse, or other qualified health professional. The testing kit typically contains a sterile wipe, a finger lancet (pricking tool), a pipette or similar suctioning device, a liquid buffer in a dropper bottle, and a disposable device called a cassette. The cassette looks like a standard home pregnancy test and has both a well into which a drop of blood is placed and a window that delivers the positive or negative reading.

To perform a rapid LFA test:

1. The cassette is removed from its wrapper and placed on a level surface. The test must be performed within an hour once the cassette is exposed to air.
2. Your finger is cleaned with the disinfectant swab.
3. The lancet makes a tiny cut on your finger.
4. A tiny sample of blood is drawn using the pipette or suctioning device.
5. One drop of blood is added to the cassette well.
6. Two drops of the buffer are added to the well.
7. Your finger is bandaged as you await the results.

In addition to positive and negative results for IgG and/or IgM antibodies, there may be invalid results in which either the placement of the lines is contradictory or no lines appear. In such cases, the test should be repeated.

Where to Get a COVID-19 Antibody Test

The COVID-19 antibody test is mainly used for research purposes through institutions, hospitals, and governmental agencies like the CDC or the National Institutes of Health (NIH). Many state and county health officials also conduct local surveillance and will typically publicize free testing days or locations on the state’s Department of Health (DOH) or Health and Human Services (HHS) website.

Testing is also available through commercial laboratories, pharmacies, and testing centers, but you may not be able to just walk in and get one. Many states require that a physician or health provider order the test before a lab is allowed to perform it.

There are exceptions, however, and a growing number of providers in certain states are offering walk-in or scheduled testing without a doctor’s order. These services are almost always paid upfront and restricted to adults 18 and over. People under 18 can only obtain testing under the instructions of a doctor.

Two of the country’s biggest lab testing companies, Quest Diagnostics and LabCorp, are actively promoting COVID-19 antibody testing to consumers.

Quest Diagnostics

• Quest’s QuestDirect Immune Response Test can be purchased online for $119 (plus a $10.30 service fee).
• Upon registration, you are provided an online questionnaire to assess your current health.
• If you are symptom-free, the test will be approved by a Quest physician and scheduled at one of 2,200 Quest labs around the country.
• After testing, the results are delivered within one to two days through the company’s secure online portal.
• If needed, you can schedule a telephonic appointment with a Quest doctor to discuss the results.

LapCorp

• The LabCorp antibody test must be ordered by a physician, either in-person or through a telehealth provider offered by your health insurer.
• You can also request the test through the independent physician service PWN Health at a cost of $10.
• There are no upfront fees for the test if it is ordered by a physician or health provider; the billing will be sent directly to your insurer.
• If you are not insured and accessed testing through PWN Health, you may be directly billed for $119.
• The actual test can be done at a LabCorp testing center or at a partner Walgreens pharmacy.

The Quest and LabCorp antibody tests are not available in all states.

Cost and Health Insurance

Under the Families First Coronavirus Response Act passed by Congress on March 14, 2020, the cost of COVID-19 antibody testing is fully covered for people enrolled in federal healthcare programs such as Medicare, Medicaid, Veteran’s Affair, TRICARE, and Children’s Health Insurance Program (CHIP). People with private health insurance or employer-sponsored insurance are also covered, although insurance companies are allowed to institute cost-sharing measures.

For people with no insurance, the act requires Medicaid to cover the cost of antibody testing, but generally only for those who meet income criteria established by the states. People who earn more than the prescribed annual income may not be covered.

While most people can obtain the test free of charge through their insurer, there are loopholes that can result in unexpected out-of-pocket costs. There are steps you can take to avoid this:

• If you have private health insurance, check that the testing site is an in-network provider. Some insurers will only pay a preset portion of the bill if you go to a costlier out-of-network provider.
• If you pay upfront at a commercial testing site, you can usually submit the bill for reimbursement to your insurer but may be faced with the same out-of-pocket expenses if the provider is out-of-network. To avoid any hassles, check that the testing site is in-network and accepts your insurance. In this way, they can submit the claim on your behalf and save you the trouble.
• If you purchase a test that doesn’t require doctor’s authorization, don’t assume that your insurer will automatically reimburse you. Unless there is prior agreement between a lab and an insurer, most insurance companies will require that a physician order the test and that the test be FDA-authorized.
• If you have no insurance, contact your state’s Medicaid office to see if you qualify for free testing and if there are any authorized testing sites in your area. If you do not qualify, you can shop around for the best prices or call your state’s Department of Health to see if free testing is being offered at any local governmental or non-governmental institution.

A Word From Verywell

The widespread testing for COVID-19 antibodies will help public health officials better understand the disease and ways to control it. Participating in community-based surveillance studies will most definitely aid in the cause. If you do decide to get tested, it is important to understand the limitations of the test and what it can and cannot tell you.

In the end, a COVID-19 antibody test can only tell you if you have been infected in the past. Testing positive does not alter the way in which the disease is treated or prevented. Until scientists find better ways to deal with COVID-10, follow the public health guidelines and take standard precautions to reduce your risk of infection, including frequent hand-washing and getting an annual flu vaccination.

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.