Standard Short Synacthen test for suspected adrenal failure
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Indication
This is performed for the investigation of adrenal insufficiency.
Contraindication
The Synacthen test gives unreliable results within 2 weeks of pituitary surgery.
Principle
Adrenal glucocorticoid secretion is controlled by adrenocorticotrophic hormone (ACTH) released by the anterior pituitary.
This test evaluates the ability of the adrenal cortex to produce cortisol after stimulation by synthetic ACTH (tetracosactide; Synacthen ). It does not test the whole pituitary-adrenal axis.
The short test assesses the ability of the adrenal gland to respond to ACTH but is not reliable within 2 weeks of pituitary surgery.
Preparation
There are no dietary restrictions for this test. This test should be performed in the morning as the cortisol responses between the morning and late afternoon may differ by as much as 100 nmol/L at 30 min sample post Synacthen.
Prednisolone should be stopped 24 hours before the Synacthen test.
Requirements
2 plain tubes
250 microgram Synacthen (1 vial)
the dose for children is 36 microgram/kg body weight up to a maximum of 250 micrograms
Procedure
0900
take 3 mL blood for cortisol
inject Synacthen iv or im
0930
take further sample for cortisol
The above definition only defines adrenal insufficiency. The definition of normality is problematic since there is considerable variation in healthy individuals and a significant overlap with patients who have adrenal insufficiency.
In ACTH deficiency the response to the short test may be normal or reduced.
The response to Synacthen is not affected by obesity.
There is no difference in cortisol response between iv & im administration.
Baseline and incremental cortisol values do NOT apply to women taking oral contraceptives or to pregnant women.
There is quite marked variation in the measurement of cortisol in the post-Syancthen samples by different laboratory methods
Sensitivity and Specificity
There are reports of patients with incipient adrenal failure with normal responses to Synacthen. The use of physiological doses eg 1 microgram may prove more useful at determining those subjects with poor responses than conventional (250 microgram) pharmacological doses.
the standard test is better for diagnosing primary adrenal insufficiency. However, whilst there is no difference in diagnostic performance between the standard and low dose tests in cases of secondary adrenal insufficiency, neither test is recommended where there is a possibility of pituitary dysfunction
