Current status of keratinized matrices in Toxicology: Comparison of hair and nails

Nails are a keratinized matrix that has been proposed as an alternative to hair to evaluate long-term and retrospective consumption of drugs of abuse and pharmaceuticals. This matrix has been gaining interest in recent years, with new studies focusing on the analysis of fingernails and/or toenails for different substances. However, nails and hair present differences in structure, growth, and incorporation pathways that may affect drug incorporation and analysis and complicate the interpretation of the results. To better understand the results in nail samples, a comparison of concentrations found in hair, fingernails, and toenails has been described in the literature for some drugs. This review unifies the results found in the literature, with special interest on studies that report paired samples from the same individuals. Differences between fingernail and toenail samples, as well as proposed cut-offs in nails, are also discussed. Definite conclusions can be reached for some drugs, but, in general, more standardized studies are needed to better understand nail results.