We often hear owners despairing about a small increase in or celebrating a small drop in ACTH results. It is important to be aware that some horses have large variations in their ACTH levels within minutes. Liphook carried out 2 ACTH tests 10 minutes apart on 34 horses. Whilst most horses didn't show much difference between the two tests, 5 horses had considerable variations, including 2 that would have had one positive and one negative result using the 29 pg/ml cut-off - which illustrates why the more recently suggested interpretation of "less than 19 pg/ml is negative for PPID, over 40 pg/ml is positive and between 19 and 40 pg/ml requires further testing" (using Immulite CIA), may be more accurate.
Equine Veterinary Journal Volume 46, Issue 1, pages 113–117, January 2014
Investigation of rhythms of secretion and repeatability of plasma adrenocorticotropic hormone concentrations in healthy horses and horses with pituitary pars intermedia dysfunction
Rendle DI, Litchfield E, Heller J, Hughes KJ
One horse, presumably with more advanced PPID (it had hypertrichosis) that wasn't being treated, had ACTH levels that ranged from around 200 to over 1000 pg/ml within 3 weeks. Whereas another horse, also with hypertrichosis and high ACTH results, had consistently similar results. The paper concludes that "caution may be required when using ACTH concentrations to assess relative improvements or deteriorations in pituitary dysfunction in response to treatment". It also states that given that PPID affects different hormone producing cells in individual horses, "it is likely that any system of classification based upon one product from melanotrophs will have inherent limitations" - i.e. just measuring ACTH may not give the full story.
There is little question that blood tests are not yet as accurate as we would like them to be, and that there will always be a grey zone for blood results in the early stages of a progressive endocrine disease. It is important to look at the clinical signs and history and not rely on blood test results. See Is it PPID or is it EMS?