Long Synacthen Test

INDICATION

Confirmation of diagnosis of hypoadrenalism.
Differentiating primary and secondary hypoadrenalism (note that measurement of basal 0900h ACTH levels is far more sensitive than cortisol response in the long synacthen test).
The first 3 samples should give the same result as the short synacthen test.

CONTRAINDICATIONS

None

SIDE EFFECTS

None

PREPARATION

Patients who have already been taking corticosteroids should have the last dose 24 hours before the start of the test. Admit the patient if there is a risk of an Addisonian crisis (virtually never). Patients with pituitary disease are usually safe if they have an intact rennin-angiotensin (aldosterone) axis. Once the test has commenced, dexamethasone will not interfere with the cortisol result. (Do not use prednisolone, which will interfere with the cortisol assay). Use 0.75 mg for 5mg prednisolone equivalent.

1 mg tetracosactrin (depot preparation). This is not the same as ordinary Synacthen!
19g cannula.
6 red top Vacutainers.
1 purple top Vacutainer for ACTH.
Syringes.
Saline flush.
METHOD
0900h insert cannula and flush
take blood for baseline cortisol and ACTH
give 1mg depot synacthen i.m.
0930h }
1000h } Take blood
1100h } for cortisol
1300h } measurement
1700h } (i.e. additional 2, 4, 8 and 24h)
0900h }

INTERPRETATION

Normal response: baseline cortisol >170 nmol/l with rise to >900 nmol/l (peak)
Samples at 9.00, 9.30 and 10.00 can be interpreted as for a short synacthen test.
Primary adrenal insufficiency: little or no response
Secondary adrenal insufficiency: some patients may show a rise in cortisol, which may be delayed (but a subnormal response does not exclude this - measure ACTH levels).
Patients with a subnormal response can still have their steroids weaned (by 1mg pred per month).

SENSITIVITY AND SPECIFICITY

More sensitive than short synacthen test for primary adrenal insufficiency (for nomogram see Burke et al 1985).

REFERENCE

Burke C.W. et al., Clin. Endo. Metab. 14, 947-976 (1985).