Utility of measurement of dexamethasone levels in the diagnostic testing for Cushing’s syndrome

From Day 1 of the 16th International Congress of Endocrinology and the Endocrine Society’s 96th Annual Meeting and Expo »

Chicago, IL – June 21, 2014

ST Sharma, JA Yanovski, SB Abraham, LK Nieman

Summary: Dexamethasone (dex) suppression tests (DST) are used for screening and differential diagnosis of Cushing’s syndrome (CS). The 1 mg overnight (LD) DST is used to diagnose CS, the dex-suppressed CRH stimulation (Dex-CRH) test to differentiate CS from pseudocushings (PCS) while the 8 mg overnight (HD) DST is used to differentiate Cushing’s disease (CD) from ectopic ACTH syndrome (EAS). Researchers assessed the utility of dex levels in improving the diagnostic accuracy of these tests and they found that low dex and high CBG levels can account for false positive (FP) DST and Dex-CRH test results. Use of a higher dex dose in pts with low dex levels can help decrease FP results.


  • This is a retrospective study of patients (pts) with CS, PCS and normal volunteers (NV) who had a dex level measured as part of LDDST, HDDST or Dex-CRH test.
  • A post-dex cortisol (F) level ≥1.8 mcg/dl in the LDDST and a 15 min post-CRH F level ≥1.4 mcg/dl in the Dex-CRH test suggested CS.
  • A ≥69% suppression of F levels in HDDST indicated CD.
  • Dex levels


  • LDDST (n=77): Post-dex F was abnormal in 44 pts, 37 of these did not have CS on follow-up.
  • Proportion of pts with low dex levels was similar in those with incorrect or correct LDDST results (P=0.7).
  • Three of 5 pts with an abnormal result and low dex levels (44-117 ng/dl) had suppressed post-dex F levels after a 2 mg overnight DST. HDDST (N=56): Results were not consistent with the final diagnosis (CD or EAS) in 13 (23%) pts.
  • Of these, 5 had low dex levels (400-1220 ng/dl).
  • Proportion of pts with low dex levels was similar between those with correct and incorrect HDDST results (P=0.5).
  • HDDST in 1 pt with ACTH-dependent CS suggested EAS (28% suppression) with low dex level.
  • IPSS indicated CD.
  • After a doubled dex dose (16 mg), F levels suppressed by 76%, changing the HDDST result to CD.
  • Dex-CRH (n=139): Results were consistent with the final diagnosis in 133 pts (74 CS, 20 NV, 39 PCS).
  • Six pts with an abnormal result had dex levels
  • Of these, repeat testing with doubled dex dose (1 mg every 6 hours) in 2 pts led to higher dex levels (610, 757 ng/dl) and normal F level in one.
  • Two pts with abnormal result were on OCPs, 1 with a known high cortisol binding globulin (CBG) level.
  • None had CS on follow-up.
  • There was no correlation between dex and post-dex F levels in LDDST, 15 min post-CRH F levels in Dex-CRH test and % suppression of F post-dex in HDDST (P=NS).

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