Nocturnal Salivary Cortisol Is an Accurate Non-Invasive Test to Assess Endogenous Hypercortisolism in Children with Obesity and a Clinical Phenotype Suspicious for Cushing's Syndrome.

Introduction: Cushing's syndrome (CS) constitutes one of the most challenging diagnostic assessments for paediatric endocrinologists. The clinical presentation of some children with exogenous obesity overlaps with those observed in hypercortisolism states. Accurate, non-invasive first-line tests are necessary to avoid false-positive results in the obese. We aimed to evaluate the diagnostic accuracy of salivary cortisol to assess endogenous hypercortisolism in children with obesity and clinical overlapping signs of CS.

Methods: Case-control study that included children aged 2-18 years, BMI-SDS ≥2.0 and a follow-up >2 years. Patients were assigned to three categories: group A, features strongly indicative of paediatric CS (growth failure combined with increasing weight); group B, features suggestive of CS (e.g., moon face and striae); and group C, less specific features overlapping with CS (e.g., hypertension, hirsutism, insulin resistance). Children in categories A and B formed the control group. Ten patients with confirmed CS were the case group. All children collected saliva samples on the same day in the morning between 7 and 8:00 a.m. (morning salivary cortisol: mSC) and at 11 p.m. (nocturnal salivary cortisol: nSC). The mSC and nSC results were used to calculate the percentage decrease of cortisol at night (%D). Main outcomes by receiver operating characteristic for nSC and the %D were sensitivity, specificity, positive (P) and negative (N) predictive values (PV) and their corresponding 95% CI. Salivary cortisol was measured by electrochemiluminescence assay (lower limit of quantification: 2.0 nmol/L).

Results: 75/112 children met the inclusion criteria, whereas 22/75 children were eligible for the control group. Only controls decreased nSC (median and interquartile range: 2.0 [2.0-2.5] nmol/L) compared to mSC (6.9 [4.8-10.4] nmol/L), p < 0.0001. A cut-off for nSC ≥8 nmol/L confirmed CS within a sensitivity: 1.0 (0.69-1.0), specificity: 1.0 (0.85-1.0), PPV: 1.0 (0.69-0.99), and NPV: 1.0(0.85-0.99), achieving a diagnostic efficiency of 100%. The cut-off obtained for %D was 50%. No child with CS had a %D ≥50%, but 6/22 children in the control group had a %D below the cut-off, resulting in a lower overall diagnostic accuracy of 81% compared to nSC.

Conclusion: Salivary cortisol at 11 p.m. is an accurate, feasible, and non-invasive first-line test to assess endogenous hypercortisolism in children with obesity and clinical suspicion of CS. The nSC was also useful in showing that the circadian rhythm of cortisol was preserved in children with exogenous obesity. In patients with nSC ≥8.0 nmol/L, other biochemical assessments and imaging studies are needed to further confirm the aetiology.