I decided to post some data about this FRER sensitivity for those of you who are like me wonders how the test reflects the actual HCG concentrations. I made serial dilutions of human recombinant HCG (Novarel), that I had some leftovers of after my IVF cycle. The dilutions were made with lab pipets, so the concentration should be pretty precise, and in the appropriate buffer (sorry for the details, that was my husband's urine) to model the amount of HCG in the urine for the test. Here are results (see photos attached too).At 5 mU/ml, which is below the threshold of 6 mU/ml ( the sensitivity, claimed by the brand), the test is very-very faint. However, I would consider it positive, because just husband's urine only gives no second line at all (I do not have photo for this, we did it some time ago with our first unsuccessful transfer, when I had chemical pregnancy). 5 mU/ml in your first morning urine is equal about 10-15 mU/ml in blood, so this is comparable to 5-4 days before your missed period. Also, compare the 5 mU/ml results with my actual results for 8 days post transfer of 6-day blastocyst (last picture), two days later at 10dpt I had blood beta-HCG level of 38.At 20 mU/ml we can see clear positive results. This is HCG level that you will have at your first day of missed period, and equals to about 40-60 mU/ml of beta-HCG in blood.And only at 100 mU/ml we see the second line at the same brightness as the control line. I would say at this HCG level the FRER test reaches it's saturation point.Also, I found out that you may use LH tests (used for ovulation detection) instead of HCG for the early results. LH tests are very sensitive, looks like even more sensitive than FRER (you can see how much darker the test line is for the 5 mU/ml sample). I could not find an information about what subunit of LH these tests are detecting, my understanding it is beta-subunit of LH which is nearly identical in structure to beta-subunit of HCG. Beta-HCG, however, has about 30 additional amino acids, so home pregnancy tests designed to pick up beta-HCG only, while home ovulation tests will pick up both. After ovulation and during pregnancy LH levels are down, so in my opinion you can easily use you LH leftover test to monitor HCG levels, at least to see how it doubles, for example.Another observation for FRER, these tests tend to get darker with time. If your HCG levels are expected to be below or around the threshold (like day 6 post IVF transfer or so), and the test line does not show up, I suggest you keep the test for another 24 h and you may see faint positive later.For those users, who claim about "false" positive FRER results. As my experiments with my husband's urine proved, if you do not have HCG, the test will be clear negative - no test line at all, only control line. If you see faint positive, which lately with another test disappeared, or if your period came, that means you had so called chemical pregnancy. It might be devastating, I understand, we had a chemical with our first failed IVF cycle. But this is a drawback of using very sensitive test, so just be prepared.To summarize my experiment: FRER is very sensitive, it shows faint positive even below claimed threshold, however good positive results are at about 20 mU/ml, or your first day of missed period. I give only 4 stars for these tests because of the idiotic curved handle - it is completely unnecessary design, makes it difficult to lay flat, and takes more space in the drawer if you want to keep the used test for future reference.