Analyzing the Accuracy of Antibody Tests

Analyzing the Accuracy of Antibody Tests: What to Know

December 1, 2020

Since the beginning of the COVID-19 pandemic, scientists and public health experts have focused on the importance of testing. Although the message is simple, there’s still been plenty of confusion around antibody tests.

Antibody tests are not intended to diagnose an active infection of COVID-19. Instead, diagnosis is accomplished by viral tests that detect the genetic material of the virus itself or by antigen tests, which detect the presence of live virus by identifying protein receptors on the outside of the virus. The most common molecular test for the virus is conducted by PCR (polymerase chain reaction) and considered highly accurate, although this requires handling by a specialized lab. Although an antigen test is not generally as accurate as a PCR test, it can be used strategically to manage the pandemic as it is cost-effective and delivers results quickly – without the need for specialized lab equipment such as a PCR instrument.

How do antibody tests work?

Antibody testing has proved to be an essential tool in the anti-pandemic toolbox; there are times and circumstances where it is appropriate to take one. When an individual contracts a virus such as SARS-CoV-2 (which causes COVID-19), the body will mount a complex immune response. As part of this response, the body creates Y-shaped proteins called antibodies — also known as immunoglobulin (Ig) — to neutralize the virus. As the immune response continues, more and more of these antibodies are produced.

Antibody production generally peaks about two weeks or more after an initial infection. For this reason, an antibody test for COVID-19 is not ideal for diagnosing the presence of the virus or determining whether a person is contagious. However, the presence of antibodies can confirm a recent or prior infection, or indicate if someone who was not symptomatic has had the virus.

Most antibody tests for COVID-19 look for two types of antibodies: immunoglobulin M (IgM) and immunoglobulin G (IgG). IgM is usually made first in response to an infection, and its presence typically indicates a recent infection. IgG is the most common antibody found in blood and may be a better indicator of long-term immunity. For this reason, Truvian’s Easy Check COVID-19 IgM/IgG™ antibody test differentiates between detecting IgM and IgG antibodies, helping providers better contextualize a positive test result.

What about the accuracy of antibody tests?

Since the beginning of the pandemic, many antibody tests have flooded the market. Unfortunately, not all of them were developed or tested with rigorous accuracy standards. Due to this, choosing an accurate and reliable test is critical.

The key measures that determine an antibody test’s accuracy are sensitivity and specificity. Sensitivity measures the ability to detect a positive case — when the antibodies are there, how often are they detected? Specificity is the ability to accurately determine a negative case — when the antibodies are not there, how often is the sample correctly determined to be negative? For example, in a recent clinical study published in The American Journal of Clinical Pathology, Truvian’s Easy Check COVID-19 IgM/IgG™ antibody test demonstrated excellent clinical performance with a sensitivity of 96.6%, specificity of 98.2%, and overall accuracy of 98.1%.

High sensitivity and specificity are crucial because they produce few false negatives and false positives, providing more accurate data about an individual’s exposure to SARS-CoV-2. It’s unlikely that any antibody test will reach 100% specificity and sensitivity, so even the most accurate tests will produce a false positive or false negative test result. For this reason, patients should consider retesting after some time has passed – especially if an accurate antibody test result is essential.

What is antibody decay?

It’s also important to note that the antibodies developed by the immune system against a particular virus can fade over time. While antibodies for some pathogens can remain viable for a long time — antibody half-life for measles and mumps has been estimated to be more than 200 years — scientists don’t yet have conclusive evidence on the lifespan of SARS-CoV-2 antibodies. Generally, antibodies for influenza and the common cold are much shorter-lived, ranging from months to a few years. Research published in The New England Journal of Medicine suggests that the antibodies developed after a mild COVID-19 infection will decay and disappear within a few months. That said, they may still offer protection. Other studies have shown them to be more durable. However, this is key: if patients take an antibody test several months after they believe they were infected, they may very well test negative. In this case, we cannot conclude that the patient did not have COVID-19 or that they do not have protection against it — only that the test could not pick up a significant amount of antibodies.

Who should get an antibody test?

Although there is still much to learn in the fight against COVID-19, health experts stress the critical role of antibody testing. Patients might want to verify that the illness they had was COVID-19 for peace of mind; having clear knowledge of a recent or prior infection can help individuals understand their exposure history. A COVID-19 antibody test can be a powerful tool in monitoring exposure for essential healthcare workers. From a population health perspective, scalable testing data can inform public health researchers in their efforts to reduce economic and social restrictions across the world. If patients are certain they had COVID-19, they may want antibody results if considering the donation of convalescent plasma or to be involved in research studies. As COVID-19 vaccine development progresses, antibody test results may play a critical role as clinicians assess patients’ immune response to the vaccine.

While there are many valid reasons to take an antibody test, it’s critical to be informed about the type of information that antibody tests can provide. Most important is selecting an antibody test with high specificity and sensitivity – and that third-party scientists and rigorous testing processes have validated those quality measurements.

 

References

  1. “Testing the Tests: COVID-19 Antibody Assays Scrutinized for Accuracy by UCSF, UC Berkeley Researchers,” UCSF, April 27, 2020
  2. “A new study evaluated the accuracy of more than 60 coronavirus antibody tests. 13 were a cut above the rest,” Business Insider, July 23, 2020
  3. “Exploring SARS-CoV-2 Antibody Testing,” Technology Networks, September 3, 2020
  4. “How Long Do I Retain Immunity,” The New York Times, September 21, 2018
  5. “How Long Do Antibodies Against COVID-19 Really Last,” BioSpace, July 22, 2020