Failure to suppress plasma cortisol to <100 nmol/L indicates the presence of
hypercortisolism
.
Suppression to <50 excludes Cushings syndrome. Suppression to levels between 50 and 100 may be normal, or may be consistent with early Cushing's.
False positives may be due to failure to take the DEX , vomiting, or malabsorption.
Hypercortisolism can be due to:
-
Cushings syndrome (due to pituitary, adrenal, or ectopic ACTH tumours)
-
Sickness or stress can cause increased cortisol production and lack of suppression.
-
Pseudo-Cushing's syndrome from alcohol abuse, or depression
Differential diagnosis of the cause of Cushing's syndrome:
A combination of clinical assessment, plasma ACTH measurement, high dose dexamethasone suppression test and possible venous catheterisation of petrosal veins is required to establish the source of the hypercortisolism.