The Performance and Reproducibility of Late-Night Salivary Cortisol Estimation by Enzyme Immunoassay for Screening Cushing Disease

Abstract

Our study aimed to establish a local reference range for late-night salivary cortisol (LNSC) using enzyme immunoassay (EIA) and to study the intra-individual reproducibility of LNSC. Prospective study involving 30 healthy subjects (HS) with body mass index (BMI) <25 kg/m2, 37 obese/overweight subjects (OS) with BMI >25 kg/m2 and 28 patients with Cushing disease (CD). Salivary sampling was performed on 2 consecutive nights and assayed by EIA. The reference range was established using LNSC values of HS, and receiver operating characteristic (ROC) curves were used to determine diagnostic cutoffs. The mean LNSC level of CD was significantly higher than HS and OS (CD: 16.96 ± 9.11 nmol/L, HS: 1.30 ± 0.95 nmol/L, and OS 1.21 ± 0.78 nmol/L). A cutoff of 2.92 nmol/L differentiated CD from HS with 100% sensitivity and 96.7 % specificity, and a cutoff of 5.04 nmol/L yielded a specificity of 100% with a sensitivity of 96.4% to distinguish CD from OS. There was more intra-individual variability in HS (55%) than in CD (49%) and OS (22%). There was no difference in the sensitivity and specificity derived from the ROCs using day 1 values or the higher of the 2 LNSCs. In our cohort, we found that LNSC assayed by EIA showed good sensitivity and specificity to screen patients suspected to have CD. Although intra-individual variability was significant, it did not hamper the diagnostic performance of the test.