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It is much more difficult to predict the outcome of a hair drug test when body or armpit hair is used. Hair on the head grows at a relatively predictable rate that is true for the vast majority of people. So, a specific length of hair (1 and a half inches) will correlate with the last 90 days of hair growth and consequently reflect drug use over the last 90 days. In that situation, you would typically be negative, since it has been over 90 days since you last used the drug.
Body and armpit hair, though, is far less consistent and is quite variable between individuals. There is no length that can be reliably said to represent the last 90 days of hair growth. Therefore, testing of body or armpit hair can detect drug use for a longer period of time since last use in some individuals. And since there is a high amount of variability, the time period for a particular individual cannot be predicted.
So, if head hair that is long enough can be found and tested, then you should be fine, but if they must use body or armpit hair, the result cannot be reliably predicted and the drug use may be detected.
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