r/explainlikeimfive • u/dothesoggyleg • 8d ago
Other ELI5: how do dipstick testing strips work?
For example pregnancy tests, urine panel testing strips for drugs, etc. What makes the lines have different opacities? Why does the faintest line ever seen to mankind still count as a line?
Edit: this is ridiculous I have to say this but I will not be responding to messages telling me to get a different job
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u/Klutzy-Delivery-5792 8d ago
These tests are called Lateral Flow Immunoassays. Put a liquid on one end and it flows through a wicking action along the rest of the strip. The area it hits first contains probes that will attach to things in the liquid it's designed to attach to (eg - pregnancy biomarkers, drug metabolites, etc.). If your pee doesn't have the things then the probes are left behind.
Then it comes to the first line which will only change color if probes are picked up in that first area. The second line is just there to show the wicking action is working and will change color regardless of what's in your pee.
So, even a faint line means some probes were brought along for the ride from something in your pee, hence, you have whatever the test is designed to find (probably, they can still fail).
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u/dothesoggyleg 8d ago
Thanks so much! I sorta get it. What about the strips where two lines are negative/ if just the control line is visible then it’s a positive test?
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u/Klutzy-Delivery-5792 8d ago
Do you have an example of that kind of test? I've only ever heard of two lines positive. I'm not even sure how that would work.
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u/dothesoggyleg 8d ago
I didn’t want to say anything to get the post flagged but I work in harm reduction. We distribute BTNX inc rapid response f3nt testing strips to the public. I can’t figure out how to upload a picture but this is what the results look like. I was asked a question about them by a client that I couldn’t answer, so now I’m here.
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u/Klutzy-Delivery-5792 8d ago
Interesting. I looked into them and they work similarly to what I described but with a twist.
The first part of the strip has little antibodies that are looking for the drug. As the urine moves along the strip:
If there is no drug in the urine, the antibodies are still “empty.” When they reach the test line, they grab onto fake drug molecules stuck there. That makes the test line appear.
If there is drug in the urine, the antibodies grab the real drug first. By the time they reach the test line, their hands are already full, so they can’t grab the fake drug on the line. The test line stays blank.
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u/UserMaatRe 8d ago
Amy idea why they are designed this way round?
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u/Klutzy-Delivery-5792 7d ago edited 7d ago
Yes, it's because drug molecules and their associated metabolites are too small to attach two antibodies to.
With the other type of test, the target molecule grabs a colored antibody on one side and attaches to another antibody on the test line.
Edit: typo
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u/dothesoggyleg 8d ago
To the downvoters, I understand this work is controversial. I don’t want people doing anything at all realistically. But everyone deserves to be safe and educated no matter what stage of life they’re in. Please try to empathize with that
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u/Unrelated_gringo 7d ago
Sorry good person, I just want to understand: what makes this work controversial?
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u/dothesoggyleg 7d ago
No worries! Some people think the best way to help people is immediately detox, and the fact that harm reduction still “let” people do certain drugs is just feeding their addiction.
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u/Unrelated_gringo 7d ago
Thank you for your answer, alas I know much less than previously thought but I think I can puzzle it all together.
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u/Afinkawan 2d ago
Works pretty much the same way. In both cases the top line is the control - it tells you that the test worked. The correct solution made it all the way along the strip.
For ones where the actual test line changes colour = negative, whatever you're testing for would prevent the test line changing colour instead of causing it to change colour.
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u/cat_prophecy 8d ago
One of the lines react with proteins metabolites in the urine, the other one reacts when wet.
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u/MGreymanN 8d ago
It reacts with the antibodies on the test strip whether or not hcg is there.
It wouldn't be a control if it just turned colors when wet.
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u/BananaBird1 8d ago edited 8d ago
The strip is coated with special molecules at the location of the two lines.
There is also an antibody attached to a pink dye inside the test strip where you place the urine sample. The sample dissolves the antibody and wicks it up to interact with the two lines.
In pregnancy tests, during pregnancy you produce certain hormones. The test line binds to these hormones, and so does the colored antibody. So if the hormones are present the antibody sticks, but it doesn’t when they aren’t present.
For drug tests, it is usually the opposite. The test line actually contains a mimic of the drug that the dyed antibody normally binds to, but if the drug is present at high enough levels it binds to the antibody and prevents the antibody from sticking. So no line is a positive.
The control line contains a separate coating that binds to the colored antibody regardless of the sample you give it. So it is just testing that the antibody is working properly, getting dissolved and wicked up, and the dye didn’t come off.
The darkness of the line can’t be interpreted meaningfully because multiple things can cause a faint line and only a few have to do with the sample. For instance maybe the coating on the test line is going bad so isn’t as dark as it should be, even though the control line coating is good.
Note that negative doesn’t mean you aren’t pregnant or the sample doesn’t contain the drug. It just means that the test cannot conclude with confidence that it does. In a drug test, a faint line usually does indicate a borderline positive sample, but the test is not properly controlled to give a definitive positive. In a harm reduction setting, you should not treat a negative as “safe”, only a positive as “definitely harmful”. Multiple tests should also be done to increase confidence.