Subclinical
Cushing's syndrome (SCS) associated with
adrenal incidentaloma is usually characterized by autonomous
cortisol secretion without overt symptoms of
Cushing's syndrome (CS). Although the diagnostic criteria for SCS differ among countries, the 1 mg
dexamethasone suppression test (DST) is essential to confirm the presence and the extent of
cortisol overproduction. Since 1995, SCS has been diagnosed in Japan based on serum
cortisol levels ≥3 μg/dL (measured by radioimmunoassay [RIA]) after a 1 mg DST. However, the increasing use of
enzyme immunoassays (EIA) instead of RIA has hindered the diagnosis of SCS because of the differing sensitivities of commercially available assays, particularly for serum
cortisol levels of around 3 μg/dL. One way to overcome this problem is to lower the
cortisol threshold level after a 1 mg DST. In the present study, we examined the clinical applicability of lowering the
cortisol threshold to 1.8 μg/dL, similar to the American Endocrine Society's guidelines for CS, by reanalyzing 119 patients with
adrenal incidentaloma. Our findings indicate that serum
cortisol levels ≥1.8 μg/dL after 1 mg DST are useful to confirm the diagnosis of SCS if both of the following criteria are met: (1) basal
ACTH level <10 pg/mL (or poor plasma
ACTH response to
corticotrophin-releasing
hormone) and (2) serum
cortisol ≥5 μg/dL at 21:00 to 23:00 h. If only one of (1) and (2) are met, we recommend that other clinical features are considered in the diagnosis of SCS, including serum
dehydroepiandrosterone sulfate levels, urine free
cortisol levels, adrenal scintigraphy, and clinical manifestation.