FAQs on Covid-19: Can it recur? Why do we need antibodies test? What is rapid diagnostic test?

— By Dr KK Aggarwal

488: Can Covid 19 recur ?

The WHO confirmed on Saturday it is investigating reports of some recovered coronavirus patients testing positive for the illness after initially testing negative.

489: Why WHO is investigating reinfections ?

The move is in response to a report from South Korea on Friday that 91 patients who had been cleared of COVID-19 and were being prepared for discharge tested positive again. Officials say that, rather than being reinfected, patients may be suffering from a “reactivated” coronavirus.

490: What is current WHO discharge policy ?

The WHO’s clinical management guidelines state that patients can be discharged from the hospital after two negative coronavirus test results at least 24 hours apart

491: Has any country seen > 2000 deaths in a day ?

11th April: US becomes first country to record over 2,000 coronavirus deaths in last 24 hours.

492: What are Serology-based tests for COVID-19 ?

Serology testing for SARS-CoV-2 is at increased demand in order to better quantify the number of cases of COVID-19, including those that may be asymptomatic or have recovered. Serology tests are blood-based tests that can be used to identify whether people have been exposed to a particular pathogen by looking at their immune response.

K8493: How do they differ from RT PCR test ?

In contrast, the RTPCR tests currently being used globally to diagnose cases of COVID-19 can only indicate the presence of viral material during infection and will not indicate if a person was infected and subsequently recovered.

494: why do we need antibodies test ?

These tests can give greater detail into the prevalence of a disease in a population by identifying individuals who have developed antibodies to the virus.

495: What are Rapid diagnostic test (RDT) ?

This is typically a qualitative (positive or negative) lateral flow assay that is small, portable, and can be used at point of care (POC). These tests may use blood samples from a finger prick, saliva samples, or nasal swab fluids. RDTs are often similar to pregnancy tests, in that the test shows the user colored lines to indicate positive or negative results.
In the context of COVID-19, these tests most frequently test for patient antibodies (IgG and IgM), or viral antigen. In some cases, it can be beneficial to measure baseline (before infection) of IgG and IgM titers.

496: What are Enzyme-linked immunosorbent assay (ELISA) tests ?

This test can be qualitative or quantitative and is generally a lab-based test. These tests usually use whole blood, plasma, or serum samples from patients. The test relies on a plate that is coated with a viral protein of interest, such as Spike protein. Patient samples are then incubated with the protein, and if the patient has antibodies to the viral protein they bind together. The bound antibody-protein complex can then be detected with another wash of antibodies that produce a color or fluorescentbased readout.

In the contest of COVID-19, these tests most frequently test for patient antibodies (IgG and IgM).

497: what are Neutralization assays ?

This test relies on patient antibodies to prevent viral infection of cells in a lab setting. Neutralization assays can tell researchers if a patient has antibodies that are active and effective against the virus, even if they have already cleared the infection. These tests require whole blood, serum, or plasma samples from the patient. Neutralization assays depend on cell culture, a lab-based method of culturing cells that allow SARS-CoV-2 growth (like VeroE6 cells).

When virus and cells are grown with decreasing concentrations of patient antibodies, researchers can visualize and quantify how many antibodies in the patient serum are able to block virus replication. This blocking action can happen through the antibody binding to an important cell entry protein on the virus.

498: Can RT PCR be false negative ?

Negative RT-PCR tests on oropharyngeal swabs despite CT findings suggestive of viral pneumonia have been reported in some patients who ultimately tested positive for SARS-CoV-2 [Radiology. 2020]

499: can RT PCR be negative and IgM positive ?

Serologic tests, as soon as generally available and adequately evaluated, should be able to identify patients who have either current or previous infection but a negative PCR test.

In one study that included 58 patients with clinical, radiographic, and epidemiologic features suspicious for COVID-19 but with negative SARS-CoV-2 PCR testing, an IgM enzyme-linked immunosorbent assay (ELISA) was positive in 93 percent (and was negative when tested separately on plasma specimens that predated the COVID-19 outbreak) [Clin Infect Dis. 2020]

500: Can we go for viral culture ?

For safety reasons, specimens from a patient with suspected or documented COVID-19 should not be submitted for viral culture.

((The writer is President, CMAAO, HCFI and Past national President IMA))