A cb digi starts at 25 miu, to get a 1-2 weeks, hence their claim you can test 4 days before your period is due....
Pregnant 1-2 weeks (25 mIU/ml),
Pregnant 2-3 weeks(200 mIU/ml)
Pregnant 3+ weeks (2000 mIU/ml)
That used to be on the cb website but I can't find it now, but this https://www.carefertility.com/ivf/viewtopic.php?p=859722&sid=df2bbfcc8177039bece2dffbff79699f Is a study that was done on them, however it questions their accuracy. Personally iv found them generally to be accurate, but do seem to vary- with my toddler I had very obvious bfps on a frer (nearly as dark as the control) and the digi still said not pregnant, this time round the day I found out my frer was quite faint, but cb digi said 1-2. Have u tested again? Xx