Screening for Cushings syndrome using the 1 mg overnight dexamethasone suppression test
L Wainwright 1, S Medbak 1 & D Meeking2
Cushings syndrome is a rare disorder of hypercortisolism with an incidence of three cases per million each year. The 1 mg overnight dexamethasone suppression test (ODST) is a commonly used screening test. Other screening tests include 24 h urinary free cortisol and diurnal variability of serum cortisol.
We carried out an audit of ODST with the intention of examining patient outcome.
Data were collected retrospectively over a two-year period from the laboratory information system using specific search terms. Clinical data were collected from request forms.
Our search found 100 possible patients. From these there were 69 patients who had undergone ODST. There were 75 tests performed in total. Of 86.6% were carried out in outpatients, 13.4% in inpatients
Fifty six test results revealed full suppression of serum cortisol (<50 nmol/l). Of 19 test results (in 13 patients) demonstrated inadequate suppression. 15.5% of ODSTs carried out in outpatients failed to suppress and 55.6% of ODSTs in inpatients failed to suppress.
Of the ODSTs that failed to suppress, one patient was subsequently diagnosed with Cushings syndrome secondary to an adrenal adenoma. Four patients were discovered to have a secondary cause for their hypercortisolism; one patient had acute renal impairment, one patient had chronic renal impairment and IHD, one patient had acute Guillain-Barré syndrome and one patient had elevated SHBG.
Five patients underwent repeat ODSTs. Two of these fully suppressed (<50 nmol/l) and three partially suppressed (<80 nmol/l) but were considered to be normal. In four patients no outcome could be established. The positive predictive value for the ODST was 7.7%.
The highest proportion of ODSTs were carried out in the Endocrine Department. This is a cheap, safe and easy test to perform. Where Cushings syndrome is suspected it is easier to exclude in an outpatient setting where there are fewer confounding medical conditions.