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In Japan, diagnostic criteria of subclinical Cushing’s syndrome (SCS) were formulated in 1996, using cutoff value for serum cortisol of 3.0 μg/dL after an overnight 1-mg dexamethasone suppression test (DST). It has made considerable contributions to the understanding and pathology of SCS. However, it has elicited two main problems: (i) the relatively low reliability of a low range of serum cortisol essential for the diagnosis by DST; (ii) different cutoff values for serum cortisol after a 1-mg DST compared with those of other countries. For the proposal of new diagnostic criteria, we conducted a retrospective cross-sectional study. 530 cases (270 men and 260 women, mean age: 60.1 ± 11.1 years) with an adrenal tumor were assessed. In the present study, the assay kits adopted for measurement of serum cortisol differed among facilities. A calibration formula was previously developed by Kuwa et al. for standardization of different kits. Using this formula, standardization of the serum cortisol values after a 1-mg DST and at night was done. As a result, using current criteria, the diagnosis of 11 out of 92 SCS cases was changed to “non-SCS”, and all of 189 non-SCS cases remained non-SCS after calibration. After all, diagnostic concordance rate before and after calibration was 96.1%, suggesting that the actual diagnosis did not change dramatically, by calibration. The cortisol values after a 1-mg DST were compared based on the presence or absence of complications (hypertension, impaired glucose tolerance or diabetes, and hypercholesterolemia). As a result, when serum cortisol after a 1-mg DST was stratified by increments of 1.0 μg/dL, the coincidental occurrence of hypertension and impaired glucose tolerance was more frequently observed in the 2-2.99 μg/dL group than in the 0-0.99 μg/dL group (39.3% vs. 23.5%, p=0.018), suggesting that the threshold for the onset of complications of adrenal tumors may be serum cortisol of 2-3μg/dL after a 1-mg DST. Under the consideration of international consistency, serum cortisol ≥1.8 μg/dL after a 1-mg DST may be useful for the detection of ‘not completely healthy’ subjects with adrenal tumor. Additionally, in the present study, almost all of the respective criteria, such as basal ACTH