Drug and alcohol biomarkers are trapped in the keratin fibers of the fingernail. Hair is also made of keratin fibers but biomarkers may be washed out of hair by common cosmetic treatments. This is a problem that doesn’t exist when testing fingernails.
Drug and alcohol biomarkers are trapped in fingernail by three main routes:
- Blood flow in the germinal matrix deposits drug/alcohol biomarkers in the nail when formed
- Blood flow in the nail bed deposits drug and alcohol biomarkers in the nail as it thickens
- Sweat and oil of the skin surrounding the nail deposits drug and alcohol biomakers into the nail
Fingernail, a keratinized protein like hair, is emerging as a popular specimen type for drug and alcohol testing. Using fingernail samples for toxicological analysis and pharmacokinetic studies has been around for decades. However, many people do not have as much experience interpreting the results as they do hair and urine and there is confusion of whether to use a clipping or scrapings. One of the most frequent questions to the laboratory concerns the window of detection.
How far back does the fingernail test go?
To answer this, we need to discuss the anatomy of the fingernail and how compounds are incorporated into the nail. Nail is keratinized protein very similar to hair. It is porous and compounds become entrapped and bind within the structure. For this discussion, we need to know 4 anatomical features: the germinal matrix, the nail plate, the nail bed, and the free edge. The nail originates at the germinal matrix and grows outward toward the fingertip. The hardened material forming the nail plate grows across the nail bed, which is rich in capillary blood flow (this causes the pinkish color to your fingernails). As the nail grows in length, material is added from underneath such that the nail lengthens and thickens, as it grows outward toward the fingertip. Once the nail plate erupts from the nail bed and extends past the end of the fingertip, the part that extends over the edge of the fingertip is called the free edge. The free edge is the piece that you clip when clipping your nails. The entire process takes up to 6 months depending on the health of the individual.
Compounds are incorporated into fingernail by four main routes which are superimposed on top of each other rendering a very complex drug history picture:
The first route of incorporation, just like hair, is environmental exposure. If someone is handling drugs or around someone smoking drugs, the drug gets on the nail and works its way into the pores and binds to the keratinized protein.
The second route of incorporation is the sweat and oil of the skin surrounding the nail which deposits drug and drug metabolites into the nail.
The third route of incorporation is the blood flow in the germinal matrix which deposits drug and drug metabolites into the nail when it is formed.
- Lastly, drug and drug metabolites are deposited to the underside of the nail plate by the blood flow in the nail bed.
For fingernail clippings there is a potential for up to a 6-month detection window.
However, just like hair and urine, a negative result is not proof of abstinence, just the lack of evidence. A positive fingernail result for most drugs may be explained by drug use at any point during the 6 months prior to the collection. Being that the clipping contains the entire drug history of the growth of the nail, a nail scraping is not necessary.