false negatives are way way more common then false positives. and just because it's an expensive brand it doesn't mean it's any better.
FREF and CB digi say they can detect pregnancy up to 6 days before a missed period, but that works only in 25% of women. (i don't know if they write this on their tests in US but in EU they defo DO).
also, test are made to test pure HCG and those detection limits you see are made by putting synthetic HCG in urine of non pregnant women and then testing.
the truth is, female body produces ALL sorts of different HCG isoforms (different forms of one same hormone) when pregnant, and not all of them are detected by a HPT, and neither by blood work unless you test for them specifically. so if your body by its nature produces more of those "undetectable" forms that FRER can't simply see because of the antibody it uses, it will take longer for it to show up positive.
different tests use different antibodies so maybe IC was able to pick up on your hormones better just because the kind of antibody it has on suits your hormones better.
next, it depends how much water you are drinking before testing..
...and have you tried to retest with an IC again?
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