Why can't you use your FMU?

[goddess25]

As the title says wondering why you can't use a FMU as a + OPK?

I did one last night that was pretty close to positive and repeated it this morning with my first wee and it was positive too...but wondering why you can't use a FMU... is it too full of bacteria from sitting in your bladder overnight?

 

[mammag]

You can't use FMU because the urine is too concentrated in the morning, which means the LH levels will show higher than they actually are, which can give you a false positive result. I usually use SMU, making sure not too drink much.