Testing for HPA suppression
HPA function testing is generally reserved for patients who become
symptomatic when withdrawing from glucocorticoids. Its purpose is to
distinguish adrenal insufficiency from glucocorticoid withdrawal
syndrome (table 3). HPA function is tested by measurement of cortisol.
Exogenous glucocorticoids interfere with the cortisol assay, so
measurements must be made at least 12 hours after the last dose of
hydrocortisone or 24 hours after the last dose of prednisolone. If this
period without glucocorticoids is not achievable then treatment should
be switched to an equivalent dose of dexamethasone (table 1) before
testing as this does not interfere with the cortisol assay.
Patients should initially undergo measurement of 9am cortisol. Values of
<100mmol/L indicate cortisol insufficiency and
>450nmol/L indicate cortisol sufficiency. Where values are
equivocal (100-449nmol/L), a stimulation test should be performed to
measure cortisol before and at 30 and 60 minutes after administration of
intramuscular synthetic ACTH (short synacthen test). A peak cortisol
value of <550nmol/L after stimulation indicates cortisol
insufficiency. Check all these reference values with your local
laboratory as assays may vary.