Dexamethasone suppression test
The dexamethasone suppression test (DST or dexamethasone suppression test ) is to exclude (not for diagnosis) used in human and veterinary medicine of Cushing's syndrome.
- 3.1 In humans
- 3.2 In animals
The test
It is a provocation test. In a healthy organism is a control circuit between the adenohypophysis (this is a part of the pituitary gland ), adrenal cortex and cortisol blood content. The latter decreases from, the pituitary gland produces more adrenocorticotropic hormone (ACTH ), which stimulates the adrenal cortex to increased cortisol production. With increased cortisol in the blood, the pituitary gland slows the production of the hormone control.
DST in a small amount of dexamethasone ( synthetically produced cortisol derivative) is administered orally or intravenously. This causes an inhibition of the cortisol production and results in prolonged application to a suppression of ACTH secretion. In a healthy organism, a significant decrease (suppression) is evidence of blood cortisol value, in the case of Cushing's syndrome but not.
As the cortisol levels strong diurnal ( circadian rhythm ) and stress-related fluctuations, several blood samples are dependent on the test required.
In humans,
The short test is mainly the exclusion of Cushing's syndrome. This is extremely unlikely to result in outright drop in cortisol values.
There is a low-dose and high-dose version. The former is administered orally with a single dose of 1-2 mg of dexamethasone, the latter with up to 8 mg dexamethasone is performed. The application is carried out in the evenings 22-23 clock.
Normally, the blood level of cortisol on two consecutive days must be determined after dexamethasone administration before and. It is important that this measurement at the same time of day, in the morning usually 8-9 Clock happens.
Occasionally, a short test with one-time measurement will be used. This is less meaningful but better achieved in the outpatient setting.
Dexamethasone - long test
The long test is used to differentiate the cause of Cushing's syndrome. A significantly delayed decrease of cortisol would indicate a failure of the pituitary- hypothalamic axis. If there is no decrease in plasma values one would suspect an autonomous cortisol production or ACTH production in a tumor.
Long test proceeds 3-4 days and is carried out with a total of 8-12 mg dexamethasone. Both the daily measurements of cortisol plasma levels, as well as the application of the drug should always be done at the same times: usually early in the measurement and in the evening the application.
If after the first day of a significant reduction in cortisol plasma levels to be measurable, the test should be terminated because of Cushing 's syndrome would be unlikely.
In animals
There is the DST as high-dose and low-dose version, as long or short protocol. With the high-dose test in dogs ( canine Cushing's syndrome), a differentiation between primary or secondary Cushing's syndrome can be tried. When Primary Cushing's syndrome a tumorous change in the adrenal cortex is believed the Secondary Cushing's syndrome is part of the pituitary gland is altered by tumor, which controls the adrenal cortex. In both cases there is an overall increased production of the hormone cortisol, coupled with far-reaching effects, for example, on metabolism and immune system. In the horse (Equine Cushing's Syndrome - ECS) is displayed only the low-dose test as a tumorous changes in the adrenal cortex (primary Cushingsyndrom ) is extremely unlikely. When Long Protocol, a third blood sample is drawn for a more precise assessment and determined the cortisol blood level.
Problems and alternatives
In humans,
A clarification of the cause of Cushing's syndrome is difficult and can not be answered adequately by the DST. A positive DST proves still no Cushing's syndrome.
As an additional diagnosis can
- CRH test
- Insulin Hypoglykämietest and
- 24 -hour cortisol in urine
Be performed. In addition, imaging techniques (X-ray, CT, MRI, scintigraphy ) can be used in a possible tumor search.
In animals
The DST is only about 80 % negative - correct. When a negative result, ie, there is a residual risk of 20 % that the animal still has a Cushing's syndrome.
Consists in a horse with suspected ECS a laminitis, the use of dexamethasone is controversial. However, the amount used in the DST dexamethasone ( approximately one twenty-fifth of the smallest therapeutic amount ) rather too low to trigger a laminitis can.
The DST is technically easier to handle and less expensive than its alternative, the determination of endogenous ACTH levels. However, must be made in two blood samples at time intervals in a fairly narrow window of time during DST, while the ACTH determination requires only a single blood sample.