It needs to be at least 95% pure.
You can get aquarium testing strips to test for purity, and/or you can mix it with some of your own blood drops (not directly on the skin). The blood must almost immediately turn a dark chocolate brown, not medium or light brown.
@faust, I learned from experience it is important to test, regardless of what's on the label. I ordered based on the specs on the website, and the SN was not pure enough, ordered another from the same maker with a different product number and same supposed level of purity and the second one was pure enough.