Yes, The TRH stim is considered the gold standard test. The single ACTH is less likely to pick up borderline cases and has more potential for false positives. Both are testing ACTH levels, but the TRH stimulation tells you if the response to stimulation is normal (PPID causes abnormal response) so requires 2 vials of blood to be pulled one before anything as a baseline, one after TRH is given.
Borderline cases will often have a normal figure in the first test (of the 2 done together for TRH test) but an elevated number in the 2nd.
If ACTH is elevated it can indicate a clear case of PPID but stress could be causing it (it is a totally normal reaction for ACTH to increase during stress) ACTH in any form should never be tested after travel, after any other procedure, etc. even abnormal routine can cause an abnormal high if they are stressy about being kept in for example. This is one of the reasons why doing a single ACTH test can give more false positives.
This is my girls (negative) TRH stim results, you can see there’s a result for each of the bloods pulled and a reference range given for what is normal.
