55.診斷庫欣氏症（Cushing’s syndrome）下列何者最可靠？
(A)早上8時皮醇（cortisol）血濃度
(B)隔夜1 mg dexamethasone抑制試驗
(C)半夜皮醇血濃度
(D)低劑量dexamethasone抑制試驗

The low-dose dexamethasone suppression test (LDDST, 0.5 mg every 6 hours for 48 hours) is highly sensitive and specific for confirming Cushing's syndrome, but its diagnostic accuracy is comparable to other first-line tests such as the overnight 1 mg dexamethasone suppression test, late-night salivary cortisol, and 24-hour urinary free cortisol, with each test having unique limitations and patient-specific considerations. [1-5] LDDST may offer improved specificity in certain populations, such as those with pseudo-Cushing's states or mild/episodic disease, but recent guidelines and consensus statements recommend using at least two different tests for confirmation, and no single test is universally superior; measuring dexamethasone levels alongside cortisol can improve accuracy. [4-7] The addition of CRH stimulation to LDDST does not significantly improve diagnostic accuracy, and the choice of confirmatory test should be tailored to clinical context, assay availability, and patient factors. [4][7-8]

Current consensus from the Endocrine Society and recent reviews in the medical literature emphasize that the diagnosis of Cushing's syndrome should rely on a combination of tests, with LDDST, overnight 1 mg DST, late-night salivary cortisol, and 24-hour urinary free cortisol all considered valid options. The selection depends on patient characteristics, assay availability, and potential confounders such as medication use, renal function, and circadian rhythm disturbances.