What Is The Test For SARS-CoV-2?

As I’m trying to dig out the problems with SARS-CoV-2 testing the United States, it’s become necessary for me to learn a bit about how the test works. I am not an expert in RNA analysis, but this is chemistry, which I do understand. I asked Stephen N. Floor, Assistant Professor in the Department of Cell and Tissue Biology at the University of California, San Francisco, some questions and to check my work. All errors and political content in this post are mine.

I am working from the CDC instructions for the kit and their information for laboratories using the kit.

From the point of view of the person being tested, samples are taken from their respiratory tract, which means having the interior of one’s mouth and nose swabbed and perhaps washed out. They might be asked to hack up some sputum.

The laboratory procedures are demanding, but standard for RNA and DNA work.

RNA is extracted from the patient’s samples. It appears to be the extractant for this step that is currently in short supply. The extractant may be TRIzol, a solution of phenol and guanidinium isothiocyanate, neither of which should be hard to supply.

A primer and standard are added to the prepared sample, which is then run through a PCR machine.

PCR stands for polymerase chain reaction, which is a method to make many copies of DNA. Because this virus is an RNA virus, its complementary DNA is produced, a dye is added that binds to the DNA, and the primer amplifies the SARS-CoV-2 selectively. Neither the virus RNA nor DNA is infectious, because they lack the rest of the virus.

The dye fluoresces, and the amount of fluorescence indicates how much DNA is produced. A control is added to give a known result, against which the SARS-CoV-2 result can be evaluated.

A test like this must be reliable – not too many false positive or negative results. False negatives are the more dangerous in this case, because they may result in an infected person moving about the community or a delay in treatment for a sick person. I haven’t been able to find statistics on false positive and negative rates for this test. The New York fact sheet has a short discussion of their effects.

If the sample from the patient is run through the procedure immediately, results can be available within several hours.

Despite administration promises, test kits continue to be in very limited supply, and the number of qualified laboratories and total tests small. (But numbers are all over the map, and the government doesn’t seem to be collecting them.) The reasons for this remain murky. It looks to me like a bad decision, possibly a number of bad decisions, were made early on, including not using the WHO kit and developing a kit to detect multiple coronaviruses rather than just SARS-CoV-2. This could be an organizational problem – I worked for an organization that felt it had to develop all its own computer codes, including payroll. That did not go well. Or it could be that Trump’s strong desire to deny the epidemic affected the judgement of people like Robert Redfield, CDC director.

People need to know if they’re infected so that they can observe quarantine or go about their business; doctors need to know so they can isolate patients and give them appropriate treatment; and we all need to know to understand the patterns of infection in society and take appropriate distancing measures. Right now, with so few tests, we have people self-quarantining, possibly without need, and people who don’t know they’re infected.

Also with PCR, the full genome of the virus can be sequenced, and that has been done in some cases. Trevor Bedford has an extremely informative Twitter account (@trvrb), where he explains what can be deduced about the spread of the virus from its genome.

The media need to ask better questions on the lack of tests, particularly of Redfield and Mike Pence:

  • Who made the decisions on which test to use? Why did they make those decisions?
  • The question on the decisions is the central one, but you might be able to get there by asking about the alleged shortages of materials. Why? Who are the suppliers? Why are they not in short supply in the countries that are testing?

My suspicion is that the shortages are a cover for protecting Trump’s delicate ego. That priority has to be dumped in favor of the health of Americans.

Update: And it looks like my suspicion is right. From NPR Fresh Air interview of Dan Diamond, a reporter for Politico:

But at the same time, Secretary Azar has not always given the president the worst-case scenario of what could happen. My understanding is he did not push to do aggressive additional testing in recent weeks, and that’s partly because more testing might have led to more cases being discovered of coronavirus outbreak, and the president had made clear – the lower the numbers on coronavirus, the better for the president, the better for his potential reelection this fall.

Bolding mine. There’s more in the interview, but this is the most direct indictment of the president.

Cross-posted at Balloon Juice


  1. The Blog Fodder · March 12, 2020

    The shortages are likely a feature not a bug (sorry). As with Mad Cow disease, if America doesn’t test for it, America doesn’t have it. I hear Trump doesn’t like the Dem’s proposal because it helps little people, not the rich. (Is he some relative of Leona Helmsley?)


  2. Linda Marple · March 12, 2020

    Thanks for the info .. I have heard no news outlets describe what the test would require of the person , if nothing else


  3. Thomas Smith · March 12, 2020

    I saw this post noting some genetic (DNA structural) problems in the CDC test kit: https://tomeraltman.net/2020/03/03/technical-problems-COVID-primers.html (there’s a later update if you go to the index of his blog, but it doesn’t change the overall story AFAICT).

    I don’t know enough biology to know how to evaluate the criticism though. But it seems like another inexplicable screwup…


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