[science for noobs]How does they screen for COVID-19? (parable ver.)

Why are there so many complaints about "indicators" (China made plus US made, famous brand too) missing COVID-19 positive samples?
I am trying to explain it with a parable without all the scientific terms.

To put it in the simplest words, as Jesus loved doing with parables, the quick test consists of three steps: (the slow one is accurate because it uses a different method, so the quick test can only be used for screening, not diagnosis)

1. Multiplication
What we are looking for is like finding a tiny clip in a library. In order to be able to make it visible from the CCTV (yes you are not allowed to go inside the library), we will put in something that makes the clip MULTIPLIES in number, so we have a higher chance of seeing (copies of) the clip.

2. Dye
Unfortunately the CCTV cannot see the tiny clips easily. So a dye (e.g. golden in color) to make the clips shine a nice catchy color on the monitor.

3. Detection of signal
Through the CCTV in the library, the glittering is spotted.

As you can see, this is really a round-about way of finding the clip. There are all sorts of ways that can go wrong. If every step results in 10% variation, the final result can have a huge quantitative variation(30% if the variation is independent and thus can be added) from a single IDENTICAL source.

And, no, the "clip" is NOT the virus itself, but a distinguished part of the virus called antigen (which your body's immune system identify the virus with). So the story goes like: a thief might have broken into the library and we know the thief has a special clip on it. We teach the CCTV to catch a signal produced by an indicator that reflects the presence of the clip after we mass produce all the small stuffs that can be found inside the library.

Yeah what the hell. No wonder the error rate can be so high, right?

p.s. I never took any biochemistry course in the college so all I know comes from self-studying. Please kindly correct me if you are an expert in the field, thank you.

題目 : 醫學 - 部落格分类 : 學術文藝

tag : covid-19

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人類有很多不同的屬性,有一兩種偏離正常(就當是十分之一吧)是很平常的。假設人有5種互相沒有關係的屬性,五個數值都是「正常」的機會只有一半左右。
但是如果五個數值之中,同時有三種是「不正常」的,那麼100人之中,就只有不到一人,堪稱「異界人」了。

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