"Like hair, fingernails and toenails are composed of a hard protein called keratin. Drugs are incorporated into nails from the blood stream and remain locked in the nail as it grows. Nails grow in both length and thickness. Drugs enter the nail from the base (cuticle end) as the keratin is formed and via the nail bed that extends under the full length of nail.
"The distal end or free end of the fingernails and toenails are clipped for testing. If length does not allow, the surface can be shaved but is not the preferred sample. If the surface is scrapped using a razor blade (a medical device) then this one procedure probably requires the collector to follow HIPAA requirements. Nail polish and acrylic nails must be removed prior to collecting the nail sample.
"The method of screening for drug use in nail tests is the same as hair, Immunoassay. The nail is put in a chemical solution to remove external contaminants and then liquefied. All drugs found in the initial screen are confirmed by one of the methods previously explained.
"Drugs can be identified in nail clippings 2-4 weeks following ingestion and can be detected from 3 to 8 months or longer. The broad range is based on numerous factors. Fingernails grow (approximately .12 inches per month) faster than toenails (approximately .042 inches per month), longer fingers grow faster than short fingers, age and gender of the person, the time of year, the food the person eats, the dominant hand grows faster than the other hand, etc."


Turnage, Jim, "Innovations in Substance Abuse Testing," presented for the State Bar of Texas (Dallax, TX: Forensic DNA & Drug Testing Services, Inc., April 17, 2011), p. 16.