BASAL MORNING CORTISOL IN PATIENTS REFERRED FOR ACTH STIMULATION TESTING: DIAGNOSTIC PERFORMANCE IN A RETROSPECTIVE...

Accurate diagnosis of adrenal insufficiency (AI) is crucial to prevent potentially life-threatening complications. Early-morning basal serum cortisol is widely used to guide further testing, its diagnostic performance varies across clinical settings.

To evaluate the diagnostic performance of early-morning basal cortisol in patients referred for ACTH stimulation testing and to determine whether basal cortisol alone can reliably exclude AI.

This retrospective cohort study included 404 adult outpatients (mean age 45.3 years; 18.8% male) who underwent standard-dose ACTH (250 µg) stimulation testing between 2018 and 2022. All consecutive patients with available early-morning basal cortisol measurements were included. AI was defined as a peak cortisol response <18 µg/dL (≈497 nmol/L). Diagnostic performance was assessed using logistic regression and receiver operating characteristic (ROC) analyses.

AI was diagnosed in 27 patients (6.7%). Basal cortisol levels were inversely associated with ACTH test failure and demonstrated moderate discriminatory ability (AUC = 0.85). Despite higher basal cortisol being associated with lower AI probability, a small proportion of patients within clinically relevant cortisol ranges still failed stimulation testing.

Early-morning basal cortisol provides valuable diagnostic information but cannot reliably exclude adrenal insufficiency when used alone. Confirmatory dynamic testing remains necessary to ensure diagnostic safety.