SARS-CoV-2 Antibody Testing: Where Are We Now?

Elizabeth Smerczak, (MLS)ASCP, MA*

Disclosures

Lab Med. 2022;53(2):e19-e29. 

In This Article

Background

On January 10th, 2020, the sequence to a novel coronavirus, SARS-CoV-2, was uploaded to Genbank.[1] At the time, it was associated with what was reported as a small outbreak of an atypical pneumonia in the Wuhan province of China.[2] The first confirmed case of this mysterious virus in the United States was reported by the Centers for Disease Control & Prevention (CDC) on January, 21, 2020.[3] On January 30, the World Health Organization (WHO) declared a public health emergency, and one day later, so did the United States.[4,5] By February 11, the WHO introduced the name of this disease: COVID-19, caused by the SARS-CoV-2 virus. Just over 1 month after the first cases had been reported, there were more than 42,000 cases in China and hundreds of suspected cases in 24 other countries.[6] On March 11, the WHO declared COVID-19 a pandemic,[4] and it was becoming increasingly clear that the world was on the brink of a public health crisis the likes of which has not been seen in any of our lifetimes.

The scope of human life lost, and devastation on many levels, is impossible to understate. At the time of this writing there have been more than 2.5 million COVID-19 deaths worldwide and more than 500,000 in the United States.[7] The economic and societal repercussions are still unfolding, and the pandemic is not over. The sacrifices and accommodations to daily life surely did have—and continue to have—an impact on slowing the spread of the SARS-CoV-2 virus and the COVID-19 disease it causes. But as the pandemic unfurled in unprecedented ways, so did the response by the medical and scientific community. Governments, private companies, educational institutions, and philanthropists joined forces in massive collaborative efforts to meet the challenges of the pandemic, all with common goals of understanding the SARS-CoV-2 virus, finding effective treatments, and developing accurate diagnostic strategies in the face of this fast-moving foe.[8,9] We need only look at the remarkable vaccine development to understand the fruits of such labor.[10,11]

We can also look to the development and distribution of diagnostic tests as a mark of achievement. Assays for SARS-CoV-2 became available to laboratories for this emerging analyte with astonishing speed and variety. The first report of a reverse transcriptase real-time polymerase chain reaction (RT-PCR) test developed with the intent of mass production was published on January 23, 2020.[12] By February 2, the WHO had 250,000 test kits delivered.[13] The CDC was granted a U.S. Food & Drug Administration Emergency Use Authorization (FDA-EUA) for an RT-PCR test on February 4 and had shipments to all 50 states the next day.[14,15] By the end of that month, there were more than 14 nucleic acid amplification tests (NAATs) commercially available.[13] This number soon skyrocketed, and NAATs quickly became the gold standard for COVID-19 diagnosis.[16] However, NAATs do have some performance issues, particularly false negative results, mostly because of difficulty in nasopharyngeal specimen collection and viral load. In addition, supply has not always been able to keep up with demand for NAATs. Antigen tests have emerged as well, but their lack of sensitivity relegates them to screening tools. Although detecting acute infection is of paramount importance, both the molecular and antigen methods have a limited detection window and cannot detect past infection.[17,18] And there are some questions they cannot answer. Early in the pandemic, little was known about the rate of transmission, asymptomatic infection, or the immunologic response to the novel virus.[19] The need to broaden testing strategies to include antibody testing (also known as serology testing) was clear, and commercially available tests followed before long.

The first SARS-CoV-2 antibody test was granted EUA on April 1, 2020.[20] Unlike NAATs, used as a primary diagnostic tool, antibody tests detect the humoral response to SARS-CoV-2 and are not to be used as a sole mode of diagnosis. They confirm past infection. As such, the FDA initially only suggested but did not require EUA for SARS-CoV-2 antibody tests.[21] Within the first week of April, there were at least 70 SARS-CoV-2 antibody tests on the market without such authorization. With them came false claims of diagnostic capabilities and false claims of FDA approval.[22] On May 5, amidst a slew of performance concerns and inappropriate marketing claims, the FDA tightened the reins on antibody tests and required EUA approval for their use.[23] But reports of underperformance persisted, and public trust was breached. In response, independent evaluations began in a collaborative effort with the FDA, the CDC, the National Institutes of Health (NIH), the National Cancer Institute (NCI), and the Biomedical Advanced Research and Development Authority (BARDA).[24] On June 19, the FDA created a "Do Not Use" list for SARS-CoV-2 antibody tests.[25] To date, more than 100 antibody tests have been removed from the market.

With better regulation and more rigorous evaluations, the list of FDA-EUA tests grew. Clinicians and laboratories still had to navigate the implementation of testing from a rapidly developed and densely populated market. Confusion and concern around performance, what the results mean, what tests can be used for, and what they say about immune status have persisted. It is now just past 1 year since the start of the COVID-19 pandemic, and much has been learned, but SARS-CoV-2 diagnostics is still a complex topic. This article focuses on 1 facet of SARS-CoV-2 testing: antibody testing. It is written from the perspective of clinical laboratory science and for the enrichment of knowledge thereof. With so many tests available, and so much research produced at breakneck speed, it is important to look back at what we have learned and to understand the SARS-CoV-2 antibody tests available, how they should and should not be used, what their limitations are, what questions they answer, and what questions remain.

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