You are correct, the best way to tell if the trilostane dose needs to be adjusted is to perform an ACTH stimulation test, 4 hours post Trilostane dose. A urine cortisol is only a screening test for Cushing's and can be increased due to stress and other diseases. I have never had a case in which I noted antibiotic interference with the absorption or metabolism of trilostane, but I have seen several cases in which dogs on antibiotics drank more water. Cushingoid dogs can also develop diabetes or kidney disease which can cause increased water intake. The skin bumps may be a skin infection resistant to antibiotics, or possibly a different type of skin infection such as demodex or ringworm (fungal skin infection), and I recommend skin scrapings for mites and bacteria +/- skin culture if bacteria are seen. Cushing's disease is not an easy disease to treat and every case is a little different, and it may not be a bad idea to get the input from an internist (www.acvim.org). I will copy a portion of a recent article by Dr. Ian Ramsey on Trilostane and urine cortisol below.
Kimberly Coyner, DVM DACVD
If the ACTH stimulation test only provides a measure of the short-term effects of trilostane then there is a need to identify a test that measures the long-term control of HAC.
Initially it would appear that the urinary corticoid to creatinine ratio (UCCR) makes
a logical choice. However, an early study reported that it was not useful, although
few details were given. Another early study demonstrated that the mean UCCR
did not decrease significantly with trilostane treatment.20 The investigators of this
study did, however, note that the UCCR was lower when the urine was collected
shortly after dosing and higher when collected later. These investigators felt that
UCCR was useful to assess the duration of effect of trilostane when collected 24 hours
after dosing, when it had the least effect. However, overall correlation with ACTH stimulation test results and clinical improvement was low in this study. In a recent
prospective study of 18 dogs that had been successfully treated with once-daily trilostane, UCCRs were monitored every 2 weeks for at least 8 weeks. Although UCCRs did decrease compared with pretreatment values, they did not fall to below the upper limit of the reference range in most dogs. Moreover, the UCCRs of 11 dogs that initially had insufficient doses of trilostane did not differ significantly from when the dosage was optimal. Post-ACTH cortisol concentrations did not correlate significantly with UCCRs at rechecks during trilostane treatment. However, UCCR could be used
with greater success to identify dogs that were being overtreated with trilostane.
These results are similar to those achieved with mitotane.
However, another recent study using twice-daily trilostane suggested that
measuring UCCR in a urine sample collected at home the same morning as a postdosing ACTH stimulation test was performed provided useful data with regard to the duration of effect of the trilostane. In many cases, this replaces a second (predosing) ACTH stimulation test being performed in dogs that had failed to respond to once daily
dosing as described by others. Further research is needed to confirm these