Late-Night Salivary Cortisol in Healthy, Community-Dwelling Asian Indians Assessed by Second-Generation ECLIA

Background: Late-night salivary cortisol (LNSC) is used as a screening test for Cushing syndrome (CS), but there is no community-derived normative data for the normal upper limit in the South Asian population. This study aimed to determine the upper limit of normal (97.5th percentile) for LNSC in an Asian Indian population using a commercially available second-generation electrochemiluminiscence immunoassay (ECLIA).

Methods: LNSC in apparently healthy community-dwelling individuals was assessed by multistage cluster sampling. Healthy individuals age 18 to 60 years from 8 urban and 8 rural clusters of Thiruvananthapuram district were studied. Thirty people from an approximate population of 1000 individuals from each cluster participated in the study. A saliva sample was collected between 11 PM and 12 midnight and analyzed using Roche COBAS-e-411 and ultrasensitive Cortisol II kits the next day.

Results: Cortisol values from 474 salivary samples were available for final analysis after exclusion of improperly collected samples. The 97.5th percentile of the LNSC concentrations was 0.25 μg/dL (6.89 nmol/L) (90% CI, 0.23-0.27 μg/dL; ie, 6.34-7.45 nmol/L). In postmenopausal women, median LNSC was significantly higher but the 90% CI for the upper limit of their LNSC (0.28μg/dL or 7.72 nmol/L) overlapped with that of premenopausal women.

Conclusions: This study establishes the normal value of LNSC estimated by second-generation ECLIA in healthy community-dwelling Asian Indian individuals for the first time. Salivary cortisol at 11 pm to 12 am is less than 0.25μg/dL (6.89 nmol/L) in the general Asian Indian population. Menopause causes a significant increase in LNSC and may lead to overdiagnosis of CS if not interpreted carefully.