Rates of adrenal insufficiency using a monoclonal vs. polyclonal cortisol assay

Objectives The diagnosis of adrenal insufficiency relies on clear cut-offs and accurate measurement of cortisol levels. Newer monoclonal antibody assays may increase the rate of diagnosis of adrenal insufficiency if traditional cortisol cut-off levels <18 mcg/dL (500 nmol/L) are applied. We aimed to determine if the rate of diagnosis of adrenal insufficiency using a 1 mcg Cosyntropin stimulation test varied with the change in cortisol assay from a polyclonal to a monoclonal antibody assay. Methods Cortisol levels obtained during the 1 mcg Cosyntropin stimulation test performed in the last semester of 2016 using a polyclonal antibody cortisol assay were compared to tests performed using a monoclonal antibody cortisol assay during the first semester of 2017. Cosyntropin tests included cortisol values obtained at baseline, 20 min and 30 min after IV administration of 1 mcg Cosyntropin. Peak cortisol cut-off value Results Stimulated cortisol values after 1 mcg Cosyntropin using the monoclonal assay in 2017 (n=38) were significantly lower (33%) compared to those obtained with the polyclonal assay in 2016 (n=27) (p-value <0.001). The number of passing tests with a peak cortisol value >18 mcg/dL fell from 74% in 2016 (20 out of 27 tests) to 29% in 2017 (11 out of 38 tests). Conclusions The change in cortisol assay substantially increased the number of patients diagnosed with adrenal insufficiency after 1 mcg Cosyntropin stimulation testing. Standardization of cortisol assays and diagnostic criteria is critical for the accurate diagnosis of adrenal insufficiency.