Cutoff values of midnight salivary cortisol for the diagnosis of overt hypercortisolism are highly influenced by methods
Introduction
The diagnosis of endogenous hypercortisolism has been considered as one of the most challenging problems in current endocrinological practice. According to recent guidelines, the diagnostic algorithm includes measurement of diurnal variation of plasma cortisol, determination of 24-hour urinary free cortisol, measurement of midnight salivary cortisol (MSC), and determination of plasma cortisol after a low dose (1 mg) dexamethasone suppression test (LDDST) [1].
Cortisol in saliva correlates well with the unbound fraction of plasma cortisol and is unaffected by changes in corticosteroid-binding globulin (CBG) levels [2]. Therefore, salivary cortisol measurement represents a convenient way to assess the concentrations of free, biologically active cortisol. Its characteristics such as non-invasive and painless sampling, and stability of samples at room temperature for weeks made it as an ideal diagnostic test. The first MSC assay was described by Umeda et al. in 1981 [3], and its diagnostic value was evaluated by Luthold et al. in 1985 [4]. Since then MSC measurement has become widely accepted and it is now recommended by the Endocrine Society Clinical Practice Guideline as a first line diagnostic test in the diagnosis of Cushing's syndrome [1].
In the majority of earlier studies MSC has been determined by radioimmunoassay (RIA) and enzyme-linked immunosorbent assay (ELISA). Although the results obtained with different assays correlated well, the absolute values showed significant discrepancies and an assay-specific reference range was needed for the best analytical sensitivity and specificity of each assay. Because only a few studies [5], [6] on commercially available automated, non-isotopic MSC assays have been reported, we determined and compared MSC using an automated electrochemiluminescent immunoassay (ECLIA, Elecsys E170, Roche Diagnostics, Switzerland, Basel) and a traditional in-house RIA, and compared the results to those obtained with LDDST to evaluate the diagnostic performance of the two assays for the diagnosis of overt hypercortisolism.
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Patients and salivary sample collection
The study involved 126 consecutive patients referred to the 2nd Department of Medicine, Semmelweis University for evaluation because of symptoms of Cushing's syndrome, obesity, or the presence of incidentally discovered adrenal adenoma. The study was approved by the local Ethical Committee. None of the patients was taking corticosteroids, oral contraceptives or other drugs interfering with the hypothalamic-pituitary-adrenal function. All patients underwent a detailed clinical, biochemical and
Midnight salivary and plasma cortisol and LDDST
As shown in Table 1, the MSC concentration determined by both methods were significantly higher in patients who proved to have Cushing's syndrome (ECLIA mean ± SE, 1.2 ± 0.41 µg/dl; RIA mean ± SD, 0.92 ± 0.38 µg/dl) compared to patients in whom Cushing's syndrome was excluded by detailed clinical, biochemical and hormonal testing (ECLIA mean ± SD, 0.2 ± 0.02 µg/dl; RIA mean ± SD, 0.28 ± 0.03 µg/dl). As expected, patients with Cushing's syndrome had significantly higher midnight plasma cortisol (23.34 ± 6.67 µg/dl)
Discussion
Salivary cortisol has been traditionally measured by RIA, or ELISA [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], and only a few studies used automated electrochemiluminescent assays or liquid chromatography tandem mass spectrometry (LC–MSMS) [20]. However, in routine clinical practice automated immunoassays are widely used, and we were unable to find a reference range of MSC for diagnosis of overt hypercortisolism using the automated ECLIA. In addition, we found only one
Acknowledgment
A.P. is a recipient of the Janos Bolyai Research Fellowship.
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