I've never been a fan of hair hair testing. Using an external surface seems problematic. While most positives will be true, you could have false positives due to environmental conditions.

This is much less likely with piss tests. The issues there are that they're typically testing for metabolites, and non-illicit substances may produce false positives, but that's easier to determine because the person typically has to have ingested something and a significant amount.... where'd they know it.