Hi Kat,
My feeling all along has been that one day off Pred before an ACTH test is insufficient.
When you consider that the biological half life of Prednisone is somewhere in the vicinity of 18 -36 hours and if "the biological half-life of a substance is the time required for half of that substance to be removed from an organism by either a physical or a chemical process....." (Wikipedia) then it is not unreasonable to deduce that not all of the steroids - (albeit a small dose), would be 'out of the system' in 24 hours. On this basis it would probably be about a week before it was substantially enough out of the system not to be registerable.
I haven't read anything particularly pertinent to this aspect but my feeling is that apart from the more severe CAH genotype/phenotypes (classics) whose levels rise extremely quickly after withdrawal of medication, a less severely affected person may need longer to stabilise without their medication before biochemical abberations may become evident.
Regarding the H1 & H2 blockers interfering with the stim test I have no idea specifically. General principal in my thinking is, the less other conflicting variables introduced, the better. Could you ask a pharmacist? You may not get an answer by tomorrow though.
