30-Minute Delta Cortisol Post-ACTH Stimulation Test and Proposed Cut-Off Levels for Adrenal Insufficiency Diagnosis

The ACTH stimulation test is used to diagnose adrenal insufficiency (AI). This study evaluated the diagnostic performance of serum delta cortisol from ACTH stimulation tests and determined appropriate cut-off levels of serum 30-minute delta cortisol for the diagnosis of AI, allowing a reduction in the number of 60-minute cortisol tests. A 6-year retrospective study in 471 patients was conducted. The performance of the serum delta cortisol in diagnosing AI was assessd using a multivariable logistic regression model and the area under ROC curves (AuROC). Both serum 30-minute and 60-minute delta cortisol demonstrated equally high diagnostic accuracy for AI (AuROC for LDT : 0.91 vs 0.90 ; HDT : 0.91 vs 0.92, respectively). The 30-minute delta cortisol test was chosen to develop proposed diagnostic cut-off levels due to its simplicity. The proposed lower cut-off level for 30-minute delta cortisol was Δ < 1.8 μg/dL for both LDT and HDT. The upper cut-off levels were Δ > 11.8 μg/dL for LDT and Δ > 10.5 μg/dL for HDT. These cut-off levels yielded high sensitivity and specificity > 90%. The 30-minute serum delta cortisol using the proposed cut-off levels provides diagnostic performance for AI equal to that of the 60-minute test and is more convenient, requires less time, less invasive and is cost-saving.