Tox Tuesday: Fingernail drug testing

August 28, 2018

If you pick a few strangers out of a crowd, most of them will have probably heard of drug tests using urine, blood or hair samples. Thanks to ongoing discussions about new forms of roadside testing, more people have heard of oral fluid testing as well. But as far as fingernail testing goes, it’s likely that many of your randomly selected strangers haven’t heard as much.

Fingernails and toenails, like hair, are made of keratinized proteins. Because these testing matrices are porous, the chemical compounds from drugs are easily bound within their structures. For both kinds of tests, the window of detection stretches several months back — about 3–6 months in fingernails, and about 8–14 months in toenails. (The time range for toenails is longer because toenails grow much slower than fingernails.)

Drug compounds can become embedded in the nail in several ways:

  • Primarily, blood containing a drug flows near the base of the nail and in the nail bed, depositing the drug in the nail as it grows both longer and thicker
  • Natural skin oil and sweat near the nail — especially in toenails
  • Exposure to drugs through handling or being around smoked drugs, which is difficult to completely erase with washes or scrubbing because of the porous nature of the fingernail

All of this creates layers of drug-use evidence in the nail.

Typically, nail samples about two to three millimeters long are clipped from each of the donor’s fingers and/or toes. It’s helpful for gathering a heavy enough sample if the donor’s nails are long enough — ideally you want to collect about 100 milligrams of sample. If the nails aren’t long enough, a sample can be shaved from the surface of the nail. From there, an immunoassay is run on the nail.

Why (or why not) use nail testing?

Why might nail testing be preferred over hair testing when the two are similar? First off — not everybody has enough hair for a test. For this reason alone, nail testing is often considered as an alternative for when hair testing isn’t an option.

Also, nails grow continuously, but hair grows in a cycle with distinct phases, which can complicate the process of interpreting drug test results and identifying exactly when drug use occurred. Some drugs may accumulate in higher concentrations in nails than in hair, like substances without nitrogen atoms.

With hair testing, a common worry is that melanin — a natural pigment that affects hair darkness — could impact drug test results, creating a hair color bias. It’s been said that this concern is eliminated with nails, but people of different ethnic backgrounds do in fact have melanin in their fingernails. A recent study published in the Journal of Analytical Toxicology points out that melanin can be found in the fingernails of people of African, Asian, Hispanic and Middle Eastern descent.

Also, unlike urine testing, there are no adulterants when using fingernails or toenails. Sample collection can be done under direct observation without invading the donor’s privacy. Unlike blood testing, there’s no need for transportation to a medical facility.

Because nail testing provides a window of detection in terms of months, it’s not the best option for detecting recent usage — within the past few days — or for detection within a very specific time frame.

The study mentioned above, “Nail analysis for drugs: A role in workplace testing?” also points out that with nail testing, it’s difficult to determine the difference between a positive result from drug consumption and one that comes from environmental exposure or handling of the drug.

Neogen offers more than 100 ELISA test kits that can detect over 300 drug analytes and metabolites. See our website for more information.


Category: Toxicology