Prednisolone, Dexamethasone and Cortisol assays
Synthetic glucocorticoids suppress ACTH and so lower serum cortisol. Indeed, this is the principle of the dexamethasone suppression test for Cushings. Short term glucocorticoid therapy probably does not significantly suppress the pituitary-adrenal axis but after medium to long term therapy, the axis is suppressed and sometimes there are circumstances when it is important to know the ability of the axis to respond to stress. At these times, stimulation testing with Synacthen, glucagon or insulin are used and it is necessary to understand the interference by the synthetic glucocorticoid.
Dexamethasone does not cross react with the cortisol assays and is not a problem. Dexamethasone has a half-life of 4-5 hours.
Prednisolone does cross react with the assays (30% with most commercially available assays). Prednisolone has a half-life of ~3.5 hours and so needs 16-18 hours to be adequately cleared from the circulation.
We would advice that prednisolone is stopped 24 hours before performing a stimulation test to ensure no interference with the cortisol assay. If glucocorticoid therapy cannot be withdrawn for so long, the use of dexamethasone would be an acceptable alternative.
Cassidy F, Ritchie JC, Verghese K, Carroll BJ. Dexamethasone metabolism in dexamethasone suppression test suppressors and nonsuppressors. Biol Psychiat 2000;47:677-680.
Rose JQ, Yurchak AM, Jusko JW. Dose dependent pharmacokinetics of prednisone and prednisolone in man. J Pharmacokin Biopharm 1981;9:389-417.
JHB 17 Nov 2009