Primary vs secondary adrenal insufficiency: ACTH-stimulated aldosterone diagnostic cut-off values by tandem mass spectrometry.

OBJECTIVES: To validate the diagnostic utility of Cortrosyn(™) stimulated aldosterone in the differentiation of primary (PAI) and secondary adrenal insufficiency (SAI) and to evaluate the effect of urine sodium levels and posture on test performance. DESIGN: Cross-sectional study. METHODS: Healthy volunteers (HV; n = 46) and patients with PAI (n = 26) and SAI (n = 29) participated in the study. Testing included cortisol and aldosterone (by liquid-chromatography tandem mass spectrometry) measurements at baseline and 30 and 60 min after 250 µg Cortrosyn(™). Plasma corticotropin (ACTH), renin activity (PRA) and urine spot sodium as a proxy for 24-h urine sodium excretion were measured at baseline. The effect of a sitting or semifowlers posture was evaluated in healthy volunteers. RESULTS: A Cortrosyn(™)-stimulated aldosterone level of 5 ng/dl (0·14 nmol/l) had 88% sensitivity and positive predictive value and 89·7% specificity and negative predictive value for distinguishing PAI from SAI. Spot urine sodium levels showed a strong correlation with peak aldosterone levels (r = -0·55, P = 0·02, n = 18) in the SAI but not PAI or HV groups. Posture did not have a significant effect on results. CONCLUSIONS: Once diagnosed with adrenal insufficiency, a stimulated aldosterone value of 5 ng/dl (0·14 nmol/l) works well to differentiate PAI from SAI. However, clinicians should be aware of the possible effect of total body sodium as reflected by spot urine sodium levels on aldosterone results. A 24-h urine sodium measurement may be helpful in interpretation.