Optimal Low Dose Synacthen Stimulation Test Sampling Time for Diagnosis of Adrenal Insufficiency Using Monoclonal Antibody Immunoassay

Context: The low-dose synacthen stimulation test (LDSST) evaluates hypothalamo-pituitary-adrenal axis function. However, studies on the correlation between morning basal and stimulated cortisol concentrations using specific monoclonal antibody immunoassays during LDSST are limited. Objective: To determine the best time points and sampling approach for cortisol measurement using mAb immunoassay in children during LDSST. Design and Setting: Morning basal and stimulated serum cortisol measurements in the LDSST were prospectively analysed in 132 children (61 girls) with clinical suspicion of adrenal insufficiency (AI). Methods: Serum cortisol concentrations were assessed at 0, 30, 40, and 60th minutes during LDSST using mAb immunoassay. Simultaneously, morning basal cortisol levels in 119 patients were measured with both mAb immunoassay and liquid chromatography-mass spectrometry. Adrenal insufficiency (AI) was defined by a peak plasma cortisol concentration below 18 μg/dL (500 nmol/L). Results: Adrenal insufficiency (AI) was excluded in 80.3% (n=106) of patients. The 40th minute showed the highest specificity (89.1%) for predicting LDSST outcomes with a single cortisol measurement. For two time-point measurements, the 40th and 60th minutes were significantly more sensitive than other possibilities (p<0.001). A morning basal cortisol level below 6.5 μg/dL was identified as predictive of a failed LDSST result. Conclusions: In LDSST, measurement of plasma cortisol at 40th and 60th minutes reduces the risk of false positivity. Single sampling at 40th minute yielded fewer false negatives than sampling at 30th or 60th minute.