Clinical Review Lesson of the week

Carbamazepine and false positive dexamethasone suppression tests for Cushing's syndrome

BMJ 2005; 330 doi: https://doi.org/10.1136/bmj.330.7486.299 (Published 03 February 2005) Cite this as: BMJ 2005;330:299
  1. Ronald C W Ma, resident specialist (rcwma@cuhk.edu.hk)1,
  2. Wing Bun Chan, medical officer1,
  3. Wing Yee So, associate consultant1,
  4. Peter C Y Tong, associate professor2,
  5. Juliana C N Chan, professor2,
  6. Chun Chung Chow, consultant1
  1. 1 Department of Medicine and Therapeutics, Prince of Wales Hospital, Shatin, New Territories, Hong Kong,
  2. 2 Department of Medicine and Therapeutics, Chinese University of Hong Kong
  1. Correspondence to: R C W Ma
  • Accepted 28 September 2004

Introduction

The clinical features of Cushing's syndrome may be subtle to begin with, and they resemble those of patients with metabolic syndrome. The key to diagnosing Cushing's syndrome is a high index of suspicion and the use of screening tests. The 1 mg overnight dexamethasone suppression test is commonly used. Subsequent biochemical evaluation often uses higher doses of dexamethasone. The overnight dexamethasone suppression test is also increasingly used in the functional evaluation of incidental adrenal masses.

We describe two patients, both of whom were suspected of having Cushing's syndrome based on abnormal results from overnight dexamethasone suppression tests. Subsequent evaluation excluded Cushing's syndrome in both patients. The false positive suppression tests were because of accelerated metabolism of dexamethasone caused by carbamazepine. Awareness of this potential pitfall in the diagnosis of Cushing's syndrome is important to avoid unnecessary investigations and anxiety.

Case reports

Case 1

A 24 year old woman was referred to the clinic for investigation of her weight gain. She had noticed progressive weight gain in recent years. She had a known history of depression, for which she has been prescribed alprazolam, sertraline, and propranolol. She had no history of steroid …

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