Things You Most Likely Didn’t Know About RATs

Peter Steven Ho
4 min readFeb 3, 2022

Something to think about the next time you get a RAT test.

Photo by Annie Spratt on Unsplash.

Everyone has seen one and there’s a good chance you’ve even used one yourself. In fact, I used a rapid antigen test (RAT) just the other day.

I said to my wife, “What a simple and ingenious little device”. And she said, “They’re not that special, they work just like the device for a home pregnancy test.”

[Microphone drops]

And, of course, she’s absolutely right!

I just hadn’t made the mental connection that they’re based on the same technology. Naturally, this piqued my curiosity and I had to find out how RATs work.

I’ve left out a lot of the technical jargon, so here’s a simple explanation.

The key component (or the “magic”) in a RAT kit is the lateral flow device (LFD). That’s the plastic device you squeeze your liquid sample into and wait for the lines to appear. The device is completely self-sufficient and it has all the required reagents in the device to do the test.

Once you add your sample to one end of the device (via the hole), a porous material wicks the sample along the strip to the other end of the device.

Sandwich assay and two line LFDs

A sandwich assay is probably the most common method used to test for COVID-19 and the LFD will have two result lines: a test (T) and a control © line.

As the liquid sample flows across the device, it encounters three different capillary pads, each doped with different reactive molecules. A fourth and final pad at the very end of the device acts as a sponge to hold any excess fluid.

Photo by John Cameron on Unsplash.
Photo by John Cameron on Unsplash.

The conjugate pad (the first pad) is laced with very specific antibodies. If the targeted antigen (viral molecule) is present, it’ll be tagged with a colour laced antibody. The analyte (a solution whose chemical constituents are as yet unidentified) will continue to flow along the strip onto the next pad.

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Peter Steven Ho

BSc (Hons), LLB. Freelance writer in Technology, Science and Travel. Come join me on a journey of discovery.