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Difficulties in Excluding Cushing Syndrome in Obese Subjects

Tran, Huy A. FACE, FRCPA, FRACP*; Petrovksy, Nikolai FRACP, PhD†

CME Review Article #1
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Obesity is currently a major public health problem affecting many countries in the Western world. One of the potential underlying causes of obesity in a minority of patients is Cushing syndrome (CS). The signs and symptoms of CS are relatively nonspecific and thus physicians rely heavily on laboratory tests for confirmation or exclusion of this diagnosis. Traditionally, the gold standard screening test for CS is a 24-hour urinary-free cortisol (UFC). However, it is known that obese subjects have high urinary cortisol excretion resulting from increased urinary clearance rate, even in the absence of true CS. We have therefore assessed the prevalence of increased UFC excretion in obese subjects as identified by a body mass index (BMI) of greater than 30 kg/m2 and found an 18.5% false-positive rate. The use of screening dexamethasone suppression tests (DSTs) is also assessed as an alternative screening strategy to the measurement of UFC for CS in obese subjects. These tests include the 1 mg, 2 mg, and 2-day low-dose dexamethasone suppression tests (LDDSTs). Our results indicate that the 2 mg overnight dexamethasone suppression test (ONDST) is simpler, more convenient, and equally accurate as the LDDST as a screening test for excluding CS in subjects with obesity. It is also more specific than the 1-mg ONDST. It should be noted, however, that there is no gold standard test for the diagnosis of CS and that no test is infallible. Unrecognized type 2 diabetes should be fully excluded before performing these dexamethasone tests in obese patients.

*Director of Clinical Chemistry and Associate Professor, Hunter Area Pathology Service, John Hunter Hospital, New South Wales, Australia; and †Director and Professor, Department of Endocrinology and Diabetes, Flinders Medical Centre, South Australia, Australia.

The authors have disclosed that they have no significant relationships with or financial interests in any commercial company that pertains to this educational activity.

Wolters Kluwer Health has identified and resolved all faculty conflicts of interest regarding this educational activity.

Reprints: Dr. Huy A Tran, Department of Clinical Chemistry, Hunter Area Pathology Service, John Hunter Hospital, NSW 2310, Australia. E-mail: huy.tran@hnehealth.nsw.gov.au.

Chief Editor's Note: This article is the 1st of 36 that will be published in 2006 for which a total of up to 36 Category 1 CME credits can be earned. Instructions for how credits can be earned precede the CME Examination at the back of this issue.

© 2006 Lippincott Williams & Wilkins, Inc.