[Lewis Thomson (OP)]

Caitlin would like help answering the following question.

"I tested positive for covid via PCR last Wednesday, I had symptoms on Tuesday which disappeared on Thursday. As a bit of an experiment in isolation I have been taking lateral flow tests to see how prevalent the virus is in my system however, the intensity of the t line seems to have gotten stronger over the weekend when I expected it to be weaker now my symptoms have ended. I wanted to know why that is the case? Are the lateral flows picking up dead virus?"

Leave your insights in the comments below...

[Bored chemist]

It's not meant to be a quantitative test.
The line might be darker, just because you are getting better at sticking a swab up your nose.
Covid doesn't have cells.

[evan_au]

An interesting experiment - if you can get your hands on the tests (they are very scarce in Australia!).

Lateral Flow Tests (LFT) or Rapid Antigen Tests (RAT) are much less sensitive than Polymerase Chain Reaction tests (PCR).
- Swabbed properly, PCR can detect that you are infected before you become infectious; and also that you have been infected when you are no longer infectious
- Swabbed properly, LFT/RAT can detect that you are infectious today (but with a higher false-negative probability)

Both tests detect parts of the virus
- PCR detects markers on the viral RNA. It can detect disintegrated virus particles that are no longer infectious, at very low levels.
- LFT/RAT detects markers on the viral protein coat. It can detect disintegrated virus particles that are no longer infectious, provided they are at quite high levels.

Possible cause of high levels after end of symptoms are:
- you are swabbing better (as per BC),
- You are keeping the test at a better temperature - if it is icy cold or uncomfortably hot, it may not work as well
- You are taking more powerful analgesics that are disguising any residual symptoms
- Mistaking "feling better" for "back to normal"
- your body is still producing some virus, even though the cold/flu symptoms have gone. This is especially a risk in immune-suppressed patients.

The traditional cold/flu symptoms occur due to your body's Interferon response
- COVID-19 manages to suppress this normal immune reaction for a couple of days, so you were  probably infectious on Sunday+Monday
- Infectious virus production tails off a few days after symptoms  improve

[Bored chemist]

Another factor to consider; if your symptoms have eased you may be producing less nasal mucous and so more of the viral protein is getting past the filter and into the test "card" because it's not getting trapped.

[set fair]

They test for specifc antigens ie parts of the virus. I think they generally look for two or three antigens, these can be on an intact viral paricle or on a fragment. For more infornation on when they test positive compared to a PCR test look up the challenge trial - they go into this in detail.