I've been reading this question a lot lately:
"Why do IC's tell you not to use FMU but CB's want you to use FMU?"
The reason for this is essentially for the same reason (high concentration of urine = high LH) but it's because the tests work in different ways. To understand why there's this different, you need to understand how the tests work.
The IC's work by the LH in your urine reacting with a chemical imbedded into the test line strip. This reaction turns the test line into a colour and the higher the concentration of LH the darker the result (it will almost always show up a little but only when the test line is as dark as the control line is it a positive result). Using FMU (or highly concentrated urine) for a test like this would risk false positives.
The CB tests work by monitoring your levels of LH and determining what your "normal" range is (which is why you re-use the monitors). Using this normal, they are able to detect when you are having a surge, or rapid increase in LH, which indicates ovulation. So, because FMU has the highest possible concentration, when that highest changes and becomes even HIGHER (as it does when you're ovulating), the monitor realises you must be ovulating, or about to, and gives a positive result.
I was using IC's but I've since decided to switch to a CB digital monitor (the one with the smilies). The reason for this is because it's easier for me to test on first waking (as opposed to remembering to test at the same time every day later in the day with an IC) and also because I found it difficult to be 100% sure if I was getting a positive or not because I often had quite dark lines.
Hope this helps!