Can positive ELISA test results better interpret for how much longer social distancing must continue?
Polymerase chain reaction (PCR) testing for SARS-CoV-2 is currently the most effective frontline test to detect exposure and infection. A positive test, taken from a nasal sample, is presumptive evidence of disease. But a negative test does not rule out infection for a variety of reasons. As many as 25% of the nasal swab tests are likely false negatives.
Causes of false-negative tests include improper collection, improper handling or storage, the prolonged time between collection and testing, or laboratory error.
Since the PCR swab test gives us no information about a person’s immunity to infection, we have been anxiously waiting for a method to test for antibodies to the virus. From experience with other infectious agents, we know that the presence of IgG antibodies indicates that immunity exists, and a presumption can often be made that the patient cannot be re-infected.
If this were, in fact, the case with SARS-CoV-2, we could send people with antibodies back to work, expecting that they will be protected. Unfortunately, not all enzyme-linked immunoassay (ELISA) tests are alike in their ability to reliably predict immunity.
Abbott Labs has been at the forefront of developing an ELISA test for COVID-19. The company has been reluctant to disclose much about its technology on the grounds that plans for deployment are still not finalized.
On March 28, Abbott announced it had received emergency clearance from the US Food and Drug Administration (FDA) to begin shipping the first point-of-care diagnostic test for the SARS-CoV-2 virus. The platform, called ID NOW, is the size of a toaster and can provide results in as little as 13 minutes. The ID NOW platform had been used to detect influenza and strep and was designed to be used by point-of-care providers.
By redesigning software and integrating different reagents, ID NOW was repurposed to test for SARS-CoV-2. Abbott said it could produce about 50,000 tests a day, with the eventual ability to produce 1.5 million tests a month. Initially, the tests were deploying only to areas with the highest concentration of COVID-19 cases.
A week later, Abbott announced it had developed an ELISA test, the m2000 SARS-CoV-2 assay, to be used in large laboratories. The company expects to produce about 4 million tests a month.
On Wednesday, Abbot announced the launch of its third test for COVID-19, a serology test to be run on its laboratory-based Architect i1000 and i2000 instruments. More than 2000 of these machines are in use in the United States. The test will cost about $5. Abbott expects to process 100-200 tests per hour. According to published reports, the number of true positives will be almost 100% within 14 days after the onset of symptoms. Remarkably, Abbott claims the false-negative rate is 99% after 14 days. It is unclear whether the test detects only IgG, or both IgG and IgM immunoglobulins.
Abbott tested its device on a few thousand patients, but when applied to millions of patients in the real world, we will see how it performs.
Another critical question is, what does a positive test tell us? Typical ELISA tests provide results for 2 distinct immunoglobulins, IgM and IgG. Typically, the IgM antibody is the first one detected. It usually rises and falls quickly, followed by a rise in IgG, which is believed to confer immunity. It may remain elevated for years, or months as in the case of influenza.
I spoke recently to Elliot A. Krauss, MD, chair of the Department of Pathology at Penn Medicine Princeton Medical Center, who expressed some concerns about the test, specifically that it didn’t undergo the same scrutiny as a test would in normal times; it was released as part of an FDA agency process that allows companies to produce and market tests without an explicit Emergency Use Authorization, although the company says it plans to file for one. As such, the test may cross-react with other common forms of coronavirus that cause the common cold.
Secondly, we don’t know which antigen is being detected by the ELISA. We know the critical antigens necessary to penetrate human cells are the s, or spike, and n antigen. There are several other antigens on the virus, some of which may be insignificant, or not enough to produce immunity, but may give rise in IgG.
Third, we don’t yet know whether the presence of IgG does, in fact, confer immunity and, if so, for how long.
Lastly, ELISA is a screening test. It casts a wide net and picks up many unrelated proteins. A positive ELISA must be confirmed by another method. Lyme disease is a prime example of a disease that causes the ELISA to be positive in many cases where there is no Lyme disease present.
Krauss had several questions about the value of the test in clinical practice. He mentioned that the University of Pennsylvania is also close to developing an antibody test, as well as other large manufacturers like Siemens and BD.
After testing positive via the ELISA test, are we going to allow teachers to return to school and doctors to return to work before we are certain they are indeed immune with no possibility of being infectious?
Krauss said it will take some time to have these questions answered.
Once again, it comes down to getting a vaccine that works, or the development of herd immunity. It is likely that the test will prove to be valuable epidemiologically and will help us better understand the natural history. It will take some time to see the real value.
For now, we will need to do both the test for active infection and the serologic test and not let the results determine whether or not to give up what we are doing. Over time we will learn the best utility of the ELISA. SARS-CoV-2 is here to stay. Most importantly, what we learn here can help us prepare for future pandemics that are undoubtedly going to affect us.
Simon Murray, MD, is an internist based in Princeton, NJ. The piece reflects his views, not necessarily those of the publication.