Latent trait cortisol (LTC) levels: reliability, validity, and stability.

The regulation of the hypothalamic pituitary adrenal (HPA) axis has received empirical attention as a mechanism contributing to individual differences in health and human development. A variety of sampling tactics and strategies index daily HPA axis functioning including the cortisol awakening response (CAR), the diurnal slope, and the area under the curve (AUGg). In an ethnically diverse sample (54% European-American, 23% Latino) of 82 adolescents (24% male, M age=18.05 years), we assessed salivary cortisol 45 times over the transition to college: 5 times per day, over 3 sequential days, across 3 waves (initially, 5, and 9 months later). Samples were collected at waking; 30 min, 3, and 8h post waking; and bedtime. Latent state-trait modeling indicated that the waking and 30 min post waking samples contributed to indices of within and across wave latent trait cortisol (LTC) levels. As such, a latent trait factor of cortisol was derived to reflect both within- and across-wave trait components of the variance in cortisol. LTC was distinct from the CAR, differentially predicted components of the diurnal profile across the day, and was highly stable across assessment waves (months). As preliminary evidence for convergent validity of LTC levels, childhood trauma was positively associated with LTC. Findings document the reliability, divergent and convergent validity, and stability of a latent trait factor of individual differences in HPA axis activity that may provide a cost efficient alternative to existing strategies and minimize participant burden.