Specificity of hair testing at standard laboratory cut-offs exceeded 90 % for all drugs , but sensitivity of hair testing relative to self-report was low , identifying only 52.3 % ( 127/243 ) of self-disclosed marijuana users , 65.2 % ( 30/46 ) of cocaine users , 24.2 % ( 8/33 ) of amphetamine users , and 2.9 % ( 2/68 ) of opioid users .