This is how it works. Trichotec did mine, though I would imagine the procedures are the same for other companies. My hair is dreadlocked, though I insisted they take the loose strands that I also have plenty of... They 'estimate' the hair grows at 'about' 1cm per month, and take the length from the tip of the hair to the root. The root being the more recent of the two. They take about a pencil width of hair, of say about 6cm. (if you have 6cm...). So, in this case a record of 6 months.
They put it through a test called ELISA, and this checks for the substances they've been asked to look for. If nothing, or a if a reading below a certain threshold, is indicated- then it is a negative result.
If the test shows up a substance above the threshold, it is put through another test called a GC/MS (or something...), which checks for the metabolites produced by substances when they are ingested. These results come in the form of a table, with the chemical names of all the different metabolites associated with all the different substances and a whole load of numbers.
If no metabolites show up in the second test, though the first indicated use, it is called a 'positive association' with that substance. Which means, basically, somehow, somewhere at some point you came into contact with that substance. So, for example, passive smoking.
The table of numbers and chemical names gives an indication of the frequency and amount of the substances over, in this case, a 6 month period.
I have lots of problems with hair strand tests. They are not accurate, and Trichotec even admitted 'hair strand testing is not an exact science.' I believe they even state this on their website. I definitely have a copy of it. I had 5 tests over a 3 year period and I got quite sick of them. The reason why I questioned the test at the time was not because I was denying using any substances, but that the tests results were inconsistent with my use. I would be happy to discuss these results openly, should anybody wish it, but they basically stated I was doing stuff when I wasn't, and wasn't when I was.
But, to the original poster, stay off the coke, bro. That metabolites very quickly into your system. If you use it with alcohol, it also produces cocethanol, another, different metabolite along with the original coke. And if you did it once over Christmas, depending on when you have your hair taken, then for the MONTH of either DEC/JAN, your results will show cocaine use as low, medium or high for that month (or, maybe both).
but, I really could go on about this forever, because they appear to me to be inaccurate and misleading at best and downright false at worse. They are just an expensive waste of time. And at the end of the day, in my case, they counted for nothing as they weren't even a consideration in the final order.
Best not do any drugs at all is the general jist methinx.