legallyblonde

I passed the At Home test from Walgreens (I'm 87 days clean, this I did to reassure myself, I get my official results on Monday), but I have an odd question. How come the presence of the pink line means NEGATIVE? Chemically, isn't the red line normally showing the presence of the tested substance? In a pregnancy test, that's how you get a positive -- it's looking for a specific substance, and the marking of the red line says you're pregnant.

So a red line in the At Home Marijuana test, even if it's faint, means negative. Why is this?


I looked over and over and yes, it's negative, but this seems to go against everything I know about stick tests in general. I'm just curious -- anybody know how these tests work?

Sec

Some tests just have a fainter line than others. If there is a line, it is negative. It has nothing to do with you, it is just the uniquness of that particular test.

You are negative for THC if you have a line, any line, in the test area.

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legallyblonde

Oh, no, I get that. I'm just curious as to why there's a chemical reaction/marker when a substance being tested for ISN'T present. Usually it's the opposite. I've taken many a pregnancy test -- which tests for a hormone that's released when the uterus detects implantation. If the hormone is present, it triggers the visible marker, and that makes much sense to me. Do you happen to know the science behind how this particular test works? Is there a counter-hormone that appears in the absence of THC-COOH -- and that is what marks?

I'm glad I'm clear -- I'm just puzzled right now because it's so counterintuitive -- especially for someone who has taken quite a few pregnancy tests! (two of which simply confirmed what I knew already!)

(I'm also one of those people who just accepts that "turn your wheels in the direction of the skid" works, even though it's counterintuitive to what you want to do when your car is out of control.)

Lateralus420

Hi legallyblonde,

Sec is absolutely right, a line does mean negative. This can seem counterintuitive, especially when compared to how pregnancy tests work. These tests are immunoassays, meaning that highly specific antibodies (designed to bind with the metabolite of interest… ex = THC metabolites in urine) are present in the test strip (where the line shows up). There are several different types of immunoassays; the drug test you speak of uses a particular strategy of detection, called a competitive immunoassay. This means that, as your urine sample travels across the test strip, it encounters colored particles that are already bound to the metabolite of interest (THC metabolites). This is why you often see a faint pink color travel across the strip with your urine before any lines appear. Imbedded within the strip, where the “THC line” should appear, there are many TCH specific antibodies. If there are NO THC metabolites in your urine, then the colored THC metabolites within the test will bind to this area, producing a pink line. If there ARE enough THC metabolites in your urine, these molecules compete for those same antibodies and bind to them instead of the colored ones in the test. This means there will be no pink line because all of the antibodies are already bound to YOUR THC metabolites (which are not bound to any colored molecules).
As for pregnancy tests, they use another type of immunoassay in which a pink line means the pregnancy hormone is present. This may be what is called a sandwich immunoassay, but I’m not sure.
This may be more info than you were looking for, but I hope I was able to explain it in an understandable fashion. If not, a google or wiki search on types of immunoassays may help clarify it.
Sorry for the long post, but once again, I hope I was able to help.
Peace.

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legallyblonde

No, that's exactly what I was looking for, and helped me to understand this a lot better. Thanks!