There isn't much that can trip it to give it a false positive. But oddly enough, there are a couple of things that react to it in the same way blood does, and this is one of them.

[ She reaches into one of her labcoat pockets, and pulls a tiny jar of...

Is that horseradish...? ]

There's an enzyme found in horseradish sauce that trips the luminol, and so any traces of it will glow, just like blood. Of course, in the field, you'd be able to tell the difference between a blood sample and horseradish pretty quickly, if the smell didn't give it away. But since it reacts in the same way, it's perfect for our purposes!