I pee on a few sticks most days to check progression even though it is not an exact science by any means. I have an endless supply of internet cheapies that will otherwise go to waste in my closet, so I've been using them up during this pregnancy, in addition to a few other sticks. It's been kind of fascinating to see the way different tests respond to HCG. For those searching for HPT progression pics, my POAS-aholism is for your viewing pleasure. ;)
The first set are my Dollar Tree tests. The interesting thing with them is that they have continued to get darker just up until about 19DPO (my beta was 1,555 that day) and then they look the same-with the control line just a hair darker than the test line. This makes me think that they hit a threshold with the dye around a beta of 1,000 and that they won't get darker anymore. They get the prize for progression with changing dye the longest.
UPDATE: 11/12/12: I continued to take the Dollar Tree tests and the test line finally became darker than the control line on 26 DPO (before that it was looking about the same darkness):
The second set are the cheap wondfo's from Amazon. You can see that by 14-15 DPO the test and control lines were equally dark and that they haven't changed since despite my HCG levels rising from 300 (13DPO) to over 1,000 by 19 DPO. Their threshold is less than Dollar Tree store tests and so while they pick up the earliest BFP's, I would say they stop showing progression with betas in the 500+ range.
The final set are the more expensive tests, the First Response Early Result (FRER). They seemed to darken up until 16 DPO, which is when the test line became darker than the control line. I like that they have the kind of dye range, but they too seem to have a threshold in the 500 beta range- as if the dye has done all it can do and simply doesn't look any darker after that.
UPDATE 11/12/12: I continued taking FRER's every other day and found that by 25 DPO, the test line looked the same darkness but the control line faded and continued to fade on 27DPO. So, these do keep changing, it just took a long time:
Anyway, it's important to note that these tests do not continue to darken indefinitely if you're using them for progression purposes (at least not until much, much later on and even then they hit a threshold!). At this point, my tests can't give me a lot of new information and the one reason to keep testing would only be to catch dramatically falling hcg. Also, please note that some days the lines looked lighter, only to darken up again...that was due to my urine being more dilute or because the dye just seemed less in that particular HPT. One day of lighter tests doesn't mean anything....it's only if they continue to lighten that you need to consider things further.
Happy stick peeing!