Nails in Forensic Toxicology: an Update

Background: The analysis of nails as a keratinized matrix to detect drugs or illicit substances has been increasingly used in forensic and clinical toxicology as a complementary test, especially for the specific characteristics of stably accumulating substances for long periods of time. This allows a retrospective investigation of chronic drug abuse, monitoring continuous drug or pharmaceutical use, reveal in utero drug exposure or environmental exposures. Methods: We herein review the recent literature investigating drug incorporation mechanisms and drug detection in nails for forensic toxicological purposes. Results: Mechanisms of drug incorporation have not yet been fully elucidated. However, some research has lately contributed to a better understanding of how substances are incorporated into nails, suggesting three potential mechanisms of drug incorporation: contamination from sweat, incorporation from nail bed and incorporation from germinal matrix. In addition, numerous methods dealing with the determination of drugs of abuse, medications and alcohol biomarkers in nails have been reported in studies over the years. The latter methods could find application in clinical and forensic toxicology. Conclusion: The studies herein reviewed point out how important it is to standardize and harmonize the methodologies (either pre-analytical or analytical) for nails analysis and the optimization of sampling as well as the development of proficiency testing programs and the determination of cut-off values.