Included in each box is a digital test reader, and sealed test sticks. You open and insert a test stick into the reader each day and test your urine, and then wait for the digital read out that comes in the form a smiley face or an empty circle.
On our first pack, we got a smiley face on the second day! I thought we could use it again next month with the same tester and the other 18 test sticks, but everything I read online seemed to say we'd have to buy a new pack. Anyways, I cracked open the digital test reader, and found a couple of batteries, and a microchip controller (Holtek HT48C06, http://www.holtek.com/english/docum/uc/48x050608_1.htm). I didn't want the results from last month to affect the reader this month, so I popped out the batteries and put them back in to hopefully reset any counter or memory in the reader.
I also bought another pack for this month so I'd have two testers (one new and one reset) to make sure everything is working as it should. And we got two smiley faces on Day 14! A couple caveats: I don't know that removing the battery actually resets anything. It could be that you could just keep using the tester and it would work just fine. A better experiment might be to have one tester that has been reset, one new tester, and one tester that has not been reset to then see how each performs, but at $30/box and ~$1 per test stick, it's an expensive experiment I'm not going to be doing any time soon. If any readers do try this out, let us know in the comments! Alternatively, if you can make sense of the technical document for the Holtek chip, likewise, we'd love to know!
Final note: while Clearblue does not sell refill packs of the test sticks, the test sticks for the Fertility Monitor seem to fit inside the OPK testers. This month we've just been using the test sticks from the OPK boxes, and if we go again next month, I'll be giving the fertility monitor test sticks a try. The Fertility Monitor Test sticks come in boxes of 30, and are about $30-$40 from Amazon.