Serum cortisol concentrations during low dose dexamethasone suppression test to screen for Cushing's syndrome.

doi:10.1136/bmj.289.6453.1188

Br Med J (Clin Res Ed) 1984;289:1188-1191 (3 November), doi:10.1136/bmj.289.6453.1188

Serum cortisol concentrations during low dose dexamethasone suppression test to screen for Cushing's syndrome.

L Kennedy, A B Atkinson, H Johnston, B Sheridan, D R Hadden

Forty four subjects (23 obese controls, 11 patients with possibleCushing's syndrome, and 10 patients with definite Cushing'ssyndrome) underwent low dose (0 X 5 mg every six hours for twodays) dexamethasone suppression tests during which serum cortisolconcentration at 0800 and excretion of urinary free cortisolover 24 hours were measured. Serum cortisol concentration fellto below 60 nmol/1 (2 X 2 micrograms/100 ml) in 31 subjectsand remained above 250 nmol/1 (9 X 1 micrograms/100 ml) in the13 others. Excretion of urinary free cortisol showed a similarresponse, falling to below 110 nmol (40 micrograms)/24 h in31 and remaining above 180 nmol (65 micrograms)/24 h in the13 others. There was complete concordance between the two variablesin terms of the pattern of response. Serum cortisol concentrationfell to below 60 nmol/1 (2 X 2 micrograms/100 ml) in at least97% (31 of a possible 32) of subjects without Cushing's syndrome.On the other hand, a serum cortisol concentration of above 250nmol/1 (9 X 1 micrograms/100 ml) after low dose dexamethasonegave a false positive diagnosis of Cushing's syndrome in atmost only one of 13 patients (7 X 7%). Measurement of serumcortisol concentration during the low dose dexamethasone testis simpler than, and as accurate and reliable as, measurementsof urinary steroids.

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