Fingernail cortisol – State of research and future directions

•Fingernail cortisol is a retrospective hypothalamic-pituitaryadrenal axis marker.•Unlike blood or salivary cortisol, it is unaffected by state-like confounders.•An advantage over hair cortisol is its high tissue availability.•Only high-intensity stress may map onto altered fingernail cortisol levels.•Little is known about trait-like confounders of fingernail cortisol. The extraction of cortisol from fingernails represents a recent advancement in the retrospective, long-term assessment of hypothalamic–pituitaryadrenal (HPA) axis activity: Fingernail cortisol has the potential to overcome some of the major disadvantages of established HPA axis markers. However, the introduction of any novel methodology also raises certain caveats. This review provides a comprehensive summary of the current state of research on fingernail cortisol. It identifies a number of strengths of his novel methodology (e.g., its high feasibility), while also pointing out open questions which currently challenge the interpretability of fingernail findings, in particular regarding the time period of cortisol secretion reflected in fingernail samples, as well as regarding potential determinants or confounders of fingernail cortisol (e.g. sociodemographic, lifestyle, or health characteristics). Clarification of these issues in the context of further methodological studies is necessary to validate the use of fingernail cortisol as a retrospective marker of HPA axis activity.