Lateral flow confusion

We took a lateral flow test this morning with what I am pretty sure was a negative result, that is to say a strong purple line at C and nothing at T. Rest of the strip white. A cassette very like but not identical to that snapped above and turned up by Bing.

That is to say, nothing at T is what you get if you look straight at the cassette in good light. If, however, you fiddle about with the viewing angle, you can pick up a very faint, narrow line at T. I thought that this was something to do with the way that the strips inside the cassettes were made, nothing to do with the test result. In any event, far fainter than that in the snap below, downloaded from the central instructions at reference 1. Nor was there any hint of colour.

But to be on the safe side, I opened up another test and took a look at an unused cassette, where I could not see any lines at all, despite fiddling with angles. And then, having opened the test, I thought we had better use it. Strong purple line at C and nothing at T. Failed to find any faint, narrow line at T. Which is still there on the first test, but now quite some time after the expiry of the 20 minute test window.

At this point, thinking that trying to phone a call centre was unlikely to be helpful, we consulted a nearby expert. Whose view was that a positive test result meant a T line that you could see, without having to search for it. Case dismissed. If you are worried, try again tomorrow. Which is where, apart from reporting the two negative results, the matter now rests.

PS: these particular cassettes were from lot COVN21110106, carried the code prefix LKG and were distributed, if not manufactured, by SureScreen Diagnostics Ltd of Prime Parkway, Derby. Snapped from Street View above.

References

Reference 1: https://www.gov.uk/government/publications/instructions-for-covid-19-self-test.

Reference 2: https://psmv5.blogspot.com/2021/12/in-praise-of-lateral-flow.html. Another sort of lateral flow.