Do what you can and take it a day at a time. I'd not worry about the testing for cushings as none seem to give any firm yes or no answers, so just stick to treating the diabetes and keeping your cat happy.
More notes on cushings as posted in the past by Dr. Lisa to another person I think....
Cushings Test Info from Dr.Lisa
You may want to ask your vet to peruse VIN if she/he is a member. I am not faulting your vet for the test that was run because it seems like this has never been very clear-cut but here are some excerpts from the VIN consultants with the date of their comments noted:
5/18/10 VIN consultant #1:
The ACTH stim has much lower sensitivity than the LDDS in cats. 60% of cats with hyperadrenocorticism return normal ACTH stim results.
That is, most cats with Cushings will not be picked up by a stim.
(LDDS = low dose dexamethasone suppression test)
6/25/10 VIN consultant #1:
The ACTH stim test is not recommended; it has very low sensitivity for Cushings in cats. The test of choice is a dexamethasone suppression test using the canine high dose.
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Other VIN consultants have recommended a UCCR...Urine Cortisol:Creatinine Ratio and while cushings can't be diagnosed simply by an elevation in the UCCR, it can be a useful tool.
The UCCR is a SENSITIVE test for Cushing's (e.g. very few animals with Cushing's would have a negative UCCR), but it is not a SPECIFIC test (e.g. many other diseases will make the UCCR elevate e.g. uncontrolled diabetes could).
So among the screening tests, we usually pick a LDDS to screen cats (using the 'high' dose for a dog DSP).
Adrenal ultrasound is also used since false positives and false negatives can be seen with the LDDS.
So, as you can see, the Dx of cushings is not straight-forward so you should probably keep going with your 'hunt' for answers....IF you and your vet are still concerned about cushings.
Lisa Pierson, dvm