With the pandemic of coronavirus affecting the world of healthcare substantially, antibody testing entered the medical world as a special opportunity to know more about the past infection, the response to vaccination, and the possible immunity.

However, what is it exactly that COVID-19 antibody test measures, and how do you interpret its results?

In this article, the author gives a clear description of COVID-19 antibody tests in regard to the type of tests, interpretation, and ranges of the test results.

What Is a COVID-19 Antibody Test?

A COVID-19 antibody test known as a serology test identifies antibodies present in your blood that form as a result of the SARS-CoV-2, virus that causes COVID-19. The antibodies are proteins of the immune system that tend to reflect previous exposure of the body to the virus or immune response to vaccines.

Two major categories of the antibodies are tested:

• IgM antibodies: tend to be the first to have evidence of after infection and will disappear with the increase of IgG. The presence of IgM can be an indication of recent infection.
• IgG antibodies: They also appear later and are likely to be present longer, sometimes months following infection or vaccination and denoted past exposure and possible immunity

How Is the Test Done?

The test is simple: a healthcare provider collects a blood sample via finger prick or vein. Results may be available in minutes (rapid test) or within a few days (lab-based tests).

Why Are COVID-19 Antibody Tests Important?

COVID-19 antibody tests serve several purposes:

• Identify Past Infection: Confirm if you have previously had COVID-19, especially if you were asymptomatic or never tested while sick.
• Monitor Vaccine Response: Respond to vaccinations by determining if antibodies are present for virus proteins that the vaccine aims to target.
• Support Epidemiology: Assist public health officials in evaluating the magnitude of infection spread within a certain population.
• Evaluate Convalescent Plasma Donation: Locate individuals that, after recovering from COVID-19, are willing to donate plasma to help severely sick individuals.

It is essential to point out that COVID-19 antibody tests do not serve the purpose of identifying active infections of the disease.

Types of COVID-19 Antibody Tests

• Qualitative Tests: Show if certain antibodies are present (positively and negatively) or absent (IgG/IgM).
• Semi-quantitative: This test applies a proprietary scale to measure the concentration of antibodies in your blood, making the results unique to that specific test and thus, incomparable to results from other testing companies.
• Quantitative/Semi-Quantitative Tests: Provide a numerical value to the concentration of IgG/IgM antibodies, estimating the level present in the sample.

Interpreting COVID-19 Antibody Test Results

The interpretation is based on the type of test, the timing of the test, the vaccination history, and the prevalence of cases in the area where the infection is.

Common Result Categories

Limitations in Interpretation

• Early testing after exposure can yield negative results due to the “window period” before antibodies develop (usually 1–3 weeks post-infection).
• Cross-reactivity with other coronaviruses can cause false positives.
• The presence of antibodies does not guarantee immunity to reinfection, nor does it measure the degree or duration of immunity.

COVID Antibody IgG Value Range / COVID Antibody Test Results Range

Quantitative COVID-19 antibody tests provide a numerical value for IgG levels, reported in Arbitrary Units per milliliter (AU/mL) or Binding Antibody Units per milliliter (BAU/mL). These values differ between labs and tests due to lack of standardization. However, some general interpretations exist:

• Example: In some studies, people had antibody levels around 1,000,000 AU/mL 1 week to 2 months after vaccination, and around 10,000 AU/mL about 3–4 months after natural infection.
• Each lab and test manufacturer sets its own “cutoff value” for a positive result. Always refer to your report’s reference range for accurate interpretation.

How to Read and Understand Your Test Report

Your antibody test report will typically state:

• Test type (qualitative or quantitative)— tells if the result is a number or simply positive/negative.
• Measured value— the actual antibody level in your blood, if quantitative.
• Reference range— indicates positive/negative cutoff specific to your test.
• Interpretation note— explains what your result likely means based on current knowledge.

Example Table: COVID-19 IgG Antibody Test Result

Note: Reference ranges (cut-offs) may vary by lab and test.

Limitations and Considerations

• Immunity: Positive result in detecting antibodies against COVID-19 does not produce immunity against proper disease and severe illness in the future. The immunity can possibly decrease with time; the reinfection has been described.
• Vaccine vs. Infection: The majority of the antibody assays will recognize antibodies produced by vaccination (generally) against the spike protein and an infection. There are some special tests able to differentiate between the two.
• Fallacious or False Results: The antibodies may cross-react to belong to other coronavirus or the test may be erroneous. False negatives may be emanated by too early testing, failure to produce antibodies, and diminishing antibodies.
• Repeat Testing: Repetitive testing: Sometimes a series of tests might be advised in progress of the research, after vaccination or suspected infection to monitor evolution.

Can an Anti-body Test Determine If You Need a Vaccine Booster?

• Current public health recommendations advise against using antibody levels alone to determine vaccine booster needs.
• Immunity is complex and involves more than measurable antibodies (e.g., T-cells). Decisions about boosters should be based on official guidelines.

Are COVID-19 antibody tests able to reveal that you have COVID-19?

No, a COVID-19 antibodies test does not reveal an acute infection. These tests show antibodies in your blood which show that you were infected or were vaccinated but do not show the virus itself, COVID-19. A molecular (RT-PCR) or antigenic test is necessary in order to know whether you are actively infected or not. The production of the antibodies normally occurs between 1 and 3 weeks after the exposure therefore a positive result indicates a past exposure and not the fact that an individual has an active infection.

How Long do COVID-19 Anti-bodies Stay in Our Body?

Antibodies of COVID 19, to be specific, the IgG antibodies can take months or even years to leave your body following infection or vaccination. This duration can also be precise with regard to immune response in patients, the severity of the infection among others. Even though these antibodies might offer some form of protection, the duration of protection and protection against a reinfection has yet to be established. The current research is still trying to determine how long the COVID-19 antibodies last and what they mean.