You don’t need to use head hair. But it’d still pick it up , because it needs a blood supply to grow even if it’s implanted.

We once tested hair for mineral content in one of my lab classes. Because I had a full beard at the time, the lab tech said it would be interesting if I tested my beard hair. Mine came back with way more sodium and potassium than the other samples. It was only later that I realised this was from eating pussy right before class. Good times.