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.