Plasma Free Cortisol in States of Normal and Altered Binding Globulins: Implications for Adrenal Insufficiency Diagnosis

Context Accurate diagnosis of adrenal insufficiency is critical because there are risks associated with overdiagnosis and underdiagnosis. Data using liquid chromatography tandem mass spectrometry (LC/MS/MS) free cortisol (FC) assays in states of high or low cortisol-binding globulin (CBG) levels, including cirrhosis, critical illness, and oral estrogen use, are needed. Design Cross-sectional. Objective Determine the relationship between CBG and albumin as well as total cortisol (TC) and FC in states of normal and abnormal CBG. Establish the FC level by LC/MS/MS that best predicts TC of <18 μg/dL (497 nmol/L) (standard adrenal insufficiency diagnostic cutoff) in healthy individuals. Subjects This study included a total of 338 subjects in four groups: healthy control (HC) subjects (n = 243), patients with cirrhosis (n = 38), intensive care unit patients (ICU) (n = 26), and oral contraceptive (OCP) users (n = 31). Main Outcome Measure(s) FC and TC by LC/MS/MS, albumin by spectrophotometry, and CBG by ELISA. Results TC correlated with FC in the ICU (R = 0.91), HC (R = 0.90), cirrhosis (R = 0.86), and OCP (R = 0.70) groups (all P < 0.0001). In receiver operator curve analysis in the HC group, FC of 0.9 μg/dL (24.8 nmol/L) predicted TC of <18 μg/dL (497 nmol/L; 98% sensitivity, 91% specificity; AUC, 0.98; P < 0.0001). Decreasing the cutoff to 0.7 μg/dL led to a small decrease in sensitivity (92%) with similar specificity (91%). Conclusions A cutoff FC of <0.9 μg/dL (25 nmol/L) in this LC/MS/MS assay predicts TC of <18 μg/dL (497 nmol/L) with excellent sensitivity and specificity. This FC cutoff may be helpful in ruling out adrenal insufficiency in patients with binding globulin derangements.