COVID testing

[Enzo]

Paging Doctor A, Paging Doctor A.

I know next to nothing about this. Please help me learn and shed misconceptions. When it comes to testing, I had always assumed the test looked for antibodies in your system. If you have them, then you were/are exposed. But it occurs to me that you could be infected but not yet developed antibodies? Meaning then that antibodies and infection are separate things? My query is that I wonder if the testing differentiates between someone who had it last month and is over it but has the antibodies and someone in whom the virus is actively raging. In other words do the tests simply look for antibodies, or do they somehow detect the presence of the virus directly?

I hope my question is clear even in the face of my lack of articulation.

[Lance]

My basic understand is that there are 2 different tests, one looking for the active infection and the other looking for antibodies.

The test for infection uses a throat swab to collect a sample, then uses PCR amplification (like that used in a DNA test) to look for the RNA of the virus itself. If positive, you have been infected. If it's early in the infection you will not yet have developed antibodies.

The second test is a blood test looking for specific antibodies. If positive, you have been infected and are fighting it. It's currently unknown if you are still shedding virus at this time, and for how long.

I'm sure Arneb will correct me and give better answers.

[Мастер]

I hope my question is clear even in the face of my lack of articulation.

Seems pretty clear to me.

Too bad I have no idea what the answer is.

[g-one]

I saw talk today about a new test for antibodies, to try and find out who has already had Covid-19.
So like Lance stated, there are 2 different kind of tests. The one that detects the presence of virus is called PCR test.

https://www.wpxi.com/news/trending/coro ... EMNYJWW3I/

[Arneb]

Lance anwered the question already. I'm just elaborating a bit.

PCR (polymerase chain reaction) tests have the advantage of being simple (PCR is off-the-shelf stuff nowadays), robust, and highly sensitive as well as specific. The SARS-CoV2 PCR test detects active viral replication (=production of virus genomes). The test can be positive in asymptomatic people, about 2 or 3 days before symptoms develop and some 4-8 days into the symptomatic phase for pharyngeal swabs, and longer than that in sputum or deep washings of the airways. It is also positive for almost as long in feces - but it is not clear if that represents undigested virus particles originally produced in the pharynx or active viral replication in the gut.

The antibody tests can only turn positive after the body has mounted a specific immune response. It detects antibodies in the patient's serum. The lag between infection, onset of symptoms and detectable antibodies is called the "diagnostic gap" and an obvious drawback for antibody testing. Others include the lower sensitivity and greater technical problems with test development and execution. Serological test for SARS-CoV2 exist, but they are not routine enough yet to apply en masse. They are used in people where the PCR is stubbornly negative in contrast to a clear clinical picture of Covid-19 or for people who present late in the disease and may already have lost viral replication.

At the moment, what you want is pick out infected people for isolation as soon as possible, even in (as yet) asymptomatic patients - that is precisely the problem PCR tests are ideal for. Later, when we ask ourselves who might have protective immunity and which proportion of infected people have remained asymptomatic, antibody testing will a have a greater role, hopefully in a test system that can be produced industrially for off-the-shelf test kits.

[MM_Dandy]

The Health Department of New York City decided to start including probable deaths in their tallies yesterday. Is it possible to retain tissue from the deceased and test it later?