I've used the pool strips but found they didn't go low enough in the pH levels to be really helpful. Flakes from her udder won't affect it but I did come in and clean her udders really well before testing. You can't get them all but you can get the majority.
Combine with pics of your mare, front, rear, sideways and pics of her teats, it's too hard to tell. She'll come soon though, that's for sure.