I use advance blue digital.. The one that gives you Low (O), High 😊, or Peak "😁" and I've been using it for months.. The first time I used a OPK i got a smile and we got pregnant. I MC 7 wks later and have been trying ever since (8-9 mons later) with no success. My cycle is pretty consistent of 32 days and I usually peak b/w day 19-21. Last cycle was unusually short of 27 days. Today is the first day I track my O and at day 15 I got a straight peak. Not so convince, I did two other test from the same urine cup and received a low (from a brand new box) and the 3rd test is cvs lines which is pretty dark but I guess it could be darker.