Testing Explained, Part I

It is highly likely that you have reached your level of saturation after five minutes of listening to the different accounts as to whether we have adequate testing (or not) for the novel Corona Virus. I will try to provide you with a basic level of understanding of what these people that you see on TV are talking about.

First, let us talk about what a normal immune system looks like. 

When you are invaded by a germ (virus; bacteria; fungus; parasite), you have several ways that you can immediately react to this challenge. There are germs that live in your skin, mouth, and gut that make chemicals that attack or make life difficult for the invaders. 

You also have cells that, even before birth, have been trained to recognize that this organism doesn’t belong to you. Once they interact with the germ, alarms, in the form of chemicals, are set off that attract other immune cells to the point of invasion. Some of these immune cells make even more chemicals to amplify the immune reaction. Others swallow the invader, to the best of their capacity, and release chemicals that cut the germ into little pieces. These pieces are then transported to the surface of the cells (the technical term is “expressed”) for still other cells to make proteins that will bind to these pieces. 

These proteins are called antibodies, or immune globulins (in the old days we called them gamma globulins).

There are four major kinds of immune globulins, but for our purposes we will only talk about two: G and M (also known as IgG and IgM). M is made first, within a few weeks, and disappears within 3-4 months. G is made later, but it sticks to the germ tighter and faster (affinity and avidity). G is responsible for long-lasting immunity since M disappears soon after it is made.

When we test for an infection, we want to know two things: which germ is causing the problem, and whether we are mounting a good immune response to it. When the invader is a bacterium, the usual process is to collect a sample of whatever tissue is causing the symptoms (for example, urine for a bladder infection). This specimen is placed in a culture medium (Petri dish, most of the time). Within two days, again most of the time, we have an answer as to which germ we are dealing with, and what antibiotic is most likely to help.

Viruses are different.

Virus cultures are expensive, technically demanding, and may take many days to give a result. Therefore, we use a technology called PCR, which makes it possible to identify the genetic material of a virus within a few minutes or hours, depending on the test. For respiratory viruses, a swab is placed deep into the nasal passage (you’d be surprised to know how much room there is back there).

As your body tries to get rid of the virus, some stragglers are expelled into the back of your throat and nose (this is called “shedding.”). The swab is manufactured so that it will catch as many viruses as possible. A plain old Q tip won’t do. Recently, shorter swabs have been released that do not have to be introduced as deeply. The swab is then placed in a test tube that needs to have certain characteristics; not any tube will do. And it must be dipped into a solution that keeps the viral material from deteriorating during transport; this is the “reagent” or “transport medium” that they talk about. Any old salt water will not do the job.

Once the tube arrives at a lab (usually what they call a reference lab; a large facility used to handling thousands of specimens a day) an expensive machine that has been taught how to do PCR for this virus processes the samples and prints out a result. Supplies of swabs, tubes, and transport medium are low, although production is ramping up. We have a population of 330 million people. If we wanted to test everyone, it is highly unlikely that there will be enough swabs, tubes, medium, or machines to do this job. This quandary is made even worse because PCR can only tell you if you have the virus today. You could have a negative test today and some stranger could sneeze close to you while you’re in line at Costco tomorrow. Now you need another test. And next week, or next month. You get the idea.

Therefore, we need to be selective as to who gets tested; this is why not everyone who wants a test should get one, at least until we can ramp up capacity to do billions of tests a month. At $100 a test…

Figure it out.

Tomorrow in Part II we will discuss immune testing.

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This Post Has 18 Comments

  1. Andrea Pacheco

    You explain things so good. Thank you. Sue Pacheco

  2. cmatre

    Great article and very helpful

  3. Shirley Edwards Greenville Mo.63944

    Sure wish you were still working in Missouri. !!! We really miss you. Please let me know if you go part time somewhere. Hope Y’all are enjoying retirement. ❤️❤️🙏🙏

  4. terry stephens

    glad to see your still around. me too !

  5. Giovanni Favignano

    Which you still my doctor. Best wishes

  6. Russ Eaves

    Good to hear from you again Paco!

  7. Verona Johnson

    Looking forward to part 2. Sounds like a very complicated process. I know you are enjoying retirement. Hope everyone in your family is doing well.

  8. Barbara Parker

    thank you for sharing

  9. Kathleen Noelker

    The highlight of my day yesterday was seeing “Paco Garriga” in my inbox. Anxiously awaiting part 2, bless you for sharing your wealth of knowledge with Neil and I.

  10. Elizabeth Townsend

    So good to “hear” from you. I think of you many times. I too wish you where still my Doc. Look forward to your talk tomorrow. thanks again.

    1. I miss you and all of my so very loyal and charming patients. Stay tuned.

  11. Cordell Webb

    So glad you are back on here. Great information. Sure wish you were still my doctor. You are the best.

  12. wiscovich9@gmail.com

    Paco, cuando explicas esta situación que estamos viviendo, todo se entiende perfectamente. Como dicen en nuestro PR, hablas en arroz y habichuelas! Gracias por la información, y por favor, sigue orientándonos. ¡ Stay home, be safe!!! Muchos cariños para ti y Phyllis.

    Migdalia