A validation study on fingernail cortisol: correlations with one-month cortisol levels estimated by hair and saliva samples

Figures & data

Table 1. Summary of previous findings about the associations of fingernail cortisol with saliva or hair cortisol.

Paper	N (male/female)	Mean age (years)	Saliva or hair collection	Fingernail collection	Results
Frugé et al. (2018)	109 (95/14)	64.1	Saliva was collected once between 8 a.m. and 2 p.m. on an assessment day.	Approximately 1–3 mm of the unattached anterior portion of the nails was clipped before the assessment day.	Fingernail cortisol in room temperature storage condition, but not in frozen storage condition, correlated with salivary cortisol (r = 0.287).
Izawa et al. (2015; Study 2)	37 (10/27)	34.5	Saliva samples were collected 8 times across two days and area under the curve of salivary cortisol was calculated.	Fingernail samples were collected for a period of 6 months and fingernail cortisol levels of each month were calculated.	Salivary cortisol correlated with cortisol levels in fingernails that were collected 3–5 months after the saliva collection (r = 0.30–0.45) but not with those collected 0–2 months after.
Izawa et al. (2015; Study 1)	58 (58/0)	56.6	Sample of 6 cm long hair from the scalp was collected on an assessment day.	Participants were asked to grow their fingernails for 2 weeks and to provide samples from every digit.	After excluding the outliers, fingernail cortisol correlated with hair cortisol (r = 0.29).

Table 2. Participants’ demographic data.

	Mean ± SD, n
Age (years)	22.7 ± 2.8
Sex (male/female)	14/9
Body mass index (kg/m^2)	20.6 ± 3.1
Smoking (smoker/non-smoker)	1/22
Hair washing (per week)	7.2 ± 1.8
Washing hands with soap (per day)	3.4 ± 3.1
Awakening time of saliva collection days^a	8:00 ± 0:50
Salivary cortisol (nmol/l)^a
Awakening	9.0 ± 2.9
+30 min	13.4 ± 5.5
Bedtime	2.9 ± 0.9
Hair cortisol (pg/mg)	9.0 ± 2.8
Fingernail cortisol (pg/mg)
0 months	6.2 ± 3.4
1 month	5.9 ± 1.7
2 months	6.1 ± 2.2
3 months	5.4 ± 1.5
4 months	6.1 ± 3.5
5 months	6.5 ± 2.3
6 months	6.6 ± 1.8
7 months	6.8 ± 3.7

^aMeans of the 30-day period.

Figure 1. Scatter plots illustrating the relationship between the cortisol levels in the 1-cm hair segment and the cortisol levels in fingernail samples collected 3 months later.

Table 3. Correlations between hair cortisol levels and fingernail cortisol levels over 8 months^a.

	Pearson’s correlation	Partial correlation^b
		Sex	Body mass index	Soap use	Hair washing
0 months	.18	.19	.19	.17	.21
1 month	.36†	.37†	.37†	.36†	.38†
2 months	.37†	.38†	.38†	.38†	.38†
3 months	.48*	.48*	.50*	.49*	.47*
4 months	.36†	.37†	.36	.37†	.38†
5 months	.32	.32	.33	.32	.35
6 months	.22	.22	.23	.22	.26
7 months	.21	.20	.21	.20	.20
All months	.36†	.37†	.36†	.36	.38†

^aCorrelations between cortisol levels in fingernails collected 0, 1, 2, 3, 4, 5, 6, and 7 months after the hair collection was demonstrated.

^bPartial correlations adjusting for sex, body mass index, frequency of washing hands with soap, or frequency of hair washing were demonstrated.

*p < .05

†p < .10.

Figure 2. Scatter plots illustrating the relationship between the 30-day integrated area under the curve based on salivary cortisol levels at awakening and bedtime (AUC_t1&3) and the cortisol levels in fingernail samples collected 2 months (A) and 5 months (B) later.

Table 4. Correlations between salivary cortisol levels (AUC_t123) and fingernail cortisol levels over 8 months^a,b.

	Pearson’s correlation	Partial correlation^c
		Sex	Body mass index	Soap use	Awakening time
0 months	−.13	−.34	−.14	−.04	−.08
1 month	.15	.08	.16	.24	.16
2 months	.35	.25	.36^†	.36	.39^†
3 months	.19	.24	.23	.35	.27
4 months	.32	.22	.32	.29	.35
5 months	.34	.28	.37^†	.41^†	.39^†
6 months	.17	.15	.20	.23	.22
7 months	.20	.16	.22	.30	.22
All months	.25	.15	.26	.33	.30

^aAUC_t123 was the 30-day summed-area under the curve calculated from the salivary cortisol levels at awakening, 30 min after awakening, and bedtime.

^bCorrelations between cortisol levels in fingernails collected 0, 1, 2, 3, 4, 5, 6, and 7 months after the saliva collection were demonstrated.

^cPartial correlations adjusting for sex, body mass index, frequency of washing hands with soap, or awakening time of saliva collection days were demonstrated.

^†p < .10.

Table 5. Correlations between baseline salivary cortisol levels (AUC_t1&3) and fingernail cortisol levels over 8 months^a,b.

	Pearson correlation	Partial correlation^c
		Sex	Body mass index	Soap use	Awakening time
0 months	−.08	−.23	−08	.00	−.03
1 month	.26	.22	.26	.36	.29
2 months	.44*	.37†	.43*	.45*	.49*
3 months	.31	.34	.30	.46*	.40†
4 months	.39†	.33	.39†	.38†	.44*
5 months	.44*	.40†	.43*	.50*	.50*
6 months	.27	.26	.26	.33	.33
7 months	.30	.27	.29	.39†	.32
All months	.35	.29	.35	.43*	.41†

^aAUC_t1&3 was the 30-day summed-area under the curve calculated from the salivary cortisol levels at awakening and bedtime.

^bCorrelations between cortisol levels in fingernails collected 0, 1, 2, 3, 4, 5, 6, and 7 months after the saliva collection were demonstrated.

^cPartial correlations adjusting for sex, body mass index, frequency of washing hands with soap, or awakening time of saliva collection days were demonstrated.

*p < .05

†p < .10.

Table 6. Intercorrelations between fingernail cortisol across 8 months.

	1 month	2 months	3 months	4 months	5 months	6 months	7 months
0 months	.34	.22	.25	.26	.27	.25	.35
1 month	–	.82**	.83**	.83**	.86**	.74**	.87**
2 months		–	.77**	.89**	.88**	.72**	.85**
3 months			–	.65**	.84**	.78**	.76**
4 months				–	.84**	.59**	.85**
5 months					–	.80**	.88**
6 months						–	.67**

**p < .01.

Frugé, A. D., Cases, M. G., Howell, C. R., Tsuruta, Y., Smith-Johnston, K., Moellering, D. R., & Demark-Wahnefried, W. (2018). Fingernail and toenail clippings as a non-invasive measure of chronic cortisol levels in adult cancer survivors. Cancer Causes & Control: CCC, 29(1), 185–191. https://doi.org/https://doi.org/10.1007/s10552-017-0989-5

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Izawa, S., Miki, K., Tsuchiya, M., Mitani, T., Midorikawa, T., Fuchu, T., Komatsu, T., & Togo, F. (2015). Cortisol level measurements in fingernails as a retrospective index of hormone production. Psychoneuroendocrinology, 54, 24–30. https://doi.org/https://doi.org/10.1016/j.psyneuen.2015.01.015

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