Quantitative Measurements of Fingernail Cortisol: Effects of Ground-fingernail Grain Size and Extraction Time

Abstract

Although cortisol levels in fingernails may reflect the hormone's long-term cumulative production, a standard measuring procedure has not yet been established. In this study, we investigated the effects of ground-fingernail grain size and cortisol extraction time on the measurement of fingernail cortisol. Fourteen healthy males provided fingernail samples, which were ground for 1, 4, or 16 minutes (coarse-, medium-, or fine-grading conditions, respectively). Subsequently, cortisol was extracted in pure methanol for 1, 6, 24, or 48 hours (four extraction time conditions). The clear supernatant was filtered and dried, and extracted cortisol was measured using an enzyme-linked immunosorbent assay. ANOVA results indicated that finer grading and longer extraction time was associated with higher cortisol levels. Furthermore, an interaction between grading and extraction-time conditions was observed: cortisol levels between medium- and fine-grading conditions were comparable in 48-hour extraction. This study clearly demonstrated that the grain-size of ground fingernails and extraction time could affect the amount of extracted cortisol obtained from fingernails and should therefore be considered during fingernail cortisol assay.