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Authors’ Response: Discriminatory Value of Low-Dose Dexamethasone Suppression Test in Establishing the Diagnosis and Differential Diagnosis of Cushing’s Syndrome

Department of Endocrinology, St. Bartholomew’s Hospital, London, United Kingdom EC1A 7BE

Address correspondence to: Dr. Ashley B. Grossman, Department of Endocrinology, St. Bartholomew’s Hospital, West Smithfield, London, United Kingdom EC1 7BE. E-mail: A.B.Grossman{at}qmul.ac.uk.

To the editor:

We read the above comments (1) with interest. As we noted in our paper (2), for the measurement of serum cortisol, we used an in-house assay in the years 1982–1998, and then, more recently, a commercial assay. The two assays were formally compared with control sera and quality controls, and the assays were found to be equivalent with no systematic bias. This was carried out on a series of patient samples assayed by the two techniques. Before 1982, we used a "Mattingley"-based fluorescence assay; this was also equivalent to the subsequent assays down to a cortisol level of 150 nmol/liter, but not below that due to nonspecific fluorescence. However, as we pointed out, there were only a handful of patients assayed with this technique, and none of the assays was used to define the absolute criterion of less than 150 nmol/liter. The question as to the equivalence of cortisol assays is an important one, and we are indeed aware of the publication cited (3) as to the problems in applying criteria based on one assay to patient groups in which a different assay is used. Indeed, in a recent review (4), we specifically stated that "diagnostic criteria that provide discrimination between groups under study are inherently reliant on the assays on which they are based. Therefore, the responses for a given test require validation in the locally used assay before they may be reliably interpreted in a given patient" (4). It is for that reason we particularly emphasized in our paper (2) the use of relative criteria (in terms of proportional falls) for the differential diagnosis of Cushing’s syndrome.

Received December 18, 2003.

References

1. Zarkovic M, Ciric J, Beleslin B, Penezic Z 2004 Discriminatory value of low-dose dexamethasone suppression test in establishing the diagnosis and differential diagnosis of Cushing’s syndrome. J Clin Endocrinol Metab 89:1486 (Letter)[Free Full Text]
2. Isidori AM, Kaltsas GA, Mohammed S, Morris DG, Jenkins P, Chew SL, Monson JP, Besser GM, Grossman AB 2003 Discriminatory value of low-dose dexamethasone suppression test in establishing the diagnosis and differential diagnosis of Cushing’s syndrome. J Clin Endocrinol Metab 88:5299–5306[Abstract/Free Full Text]
3. Clark PM, Neylon I, Raggatt PR, Sheppard MC, Stewart PM 1998 Defining the normal cortisol response to the short Synacthen test: implications for the investigation of hypothalamic-pituitary disorders. Clin Endocrinol (Oxf) 49:287–292[CrossRef][Medline]
4. Newell-Price J, Trainer P, Besser M, Grossman A 1998 The diagnosis and differential diagnosis of Cushing’s syndrome and pseudo-Cushing’s states. Endocr Rev 19:647–672[Abstract/Free Full Text]

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