HPTs

This video from TED ED shows how they work with easy to understand graphics.

Pregnancy tests are sort of complicated. There are designed to only pick up HCG, which is the pregnancy hormone. HCG should only be in your system if you are pregnant, however it will be present when taking medications that contain HCG, (like the hcg diet, and fertility treatments using the trigger shot), or with certain cancers.

Basically there are only two lines on a normal test, one that has antibodies that attach to the HCG, and another that has antibodies that attach to something else. The urine first gets mixed with the antibodies and the dye, then when it flows over the lines with more of the same antibodies (in their respective spaces) it will stick and develop the line if there is HCG present. The dye should always stick to the control line since it needs to show that it is reacting the way it should.

Source and explaination here



 This should help in explaining why lines just anywhere do not mean you are pregnant, and how evaps or indents happen. Unfortunately a line is not always positive.

An indent is just where the antibody strips are. If you looked close enough, you'd see them on a dry strip. These can be magnified while wet. 

An evap is after the tests sit too long. After the processing time (usually 10 min) the dye can settle back towards the test line. When the urine evaporates, this is more noticeable. 

Dye run happens when the test doesn't have enough urine to run across the whole test and leaves streaks, or its over-saturated and the dye pools where it shouldn't. These types of lines will be wherever they want. Usually off to the side, so the line isn't in the right spot, but sometimes they end up right over the antibody strip.

The way blue dye tests are set up is slightly different than the pinks, and they are far more susceptible to false lines. I will update when I have more information on what causes this.

*Just because this can happen, or doesn't always happen, wont rule you out as a victim of on of these lines. No one is immune since its the particular test and ALL of the circumstances that cause this. Even if you NEVER EVER got a false positive on ClearBlue before, doesn't mean that this time there is no way its an evap. This is just an example. Its the same for every test, no matter how long you've used it without incident. I have seen false lines on every single test, with personal experience with ClearBlue, Answer, and Wondfo.