Testing and monitoring for PPID

There is currently no perfect commercially available test to diagnose your horse for PPID, but your horse's age and the signs it is displaying are used to determine the best test for each individual horse.

Your veterinarian will use available guidelines set out by endocrinology conferences, researchers and special interest groups to formulate plans to diagnose and monitor PPID.

As PPID is a progressive neurodegenerative condition there is a clear distinction between what is normal and abnormal for the condition but there is a grey- zone in the middle.

In your horse's early stages of PPID, the results of basic tests may be uncertain or ambiguous. As a result, more sensitive tests are required to determine if abnormal pituitary function is contributing to vague clinical signs such as your horse's:

decreased athletic performance
loss of muscle mass
lethargy
weight loss or
elevated liver enzymes.

Older horses can be tested for endogenous ACTH concentration which is equally sensitive to the dexamethasone suppression test, but simpler and safer to perform. To be tested, these horses need to display at least one characteristic clinical sign of PPID including:

retained long hair coat (hirsutism/hypertrichosis)
loss of muscle mass
laminitis
lethargy
polyuria/polydipsia
recurrent infections
infertility, and either abnormal fat distribution in the earlier stages and weight loss in the later stages

can be tested for endogenous ACTH concentration which is equally sensitive to the dexamethasone suppression test, but simpler and safer to perform.

However, based on post mortem studies of horses, both of these tests may miss early or subclinical cases of PPID.1

Read the full article in the Clinical Review.

Endogenous ACTH concentrations

Collecting a single blood sample to measure ACTH concentration

Collection of a single blood sample for measurement of ACTH concentration requires a single trip to the farm, carries no risk of laminitis, but can be relatively expensive depending on the laboratory. Using human laboratories and reference ranges from textbooks can be problematic due to differences in assays that are used between laboratories. For example, ELISAs, immunoantibody assays and radioimmunoassays may give different results for the same animal, and even kits made by different manufacturers can give slightly different values, thus resulting in assay dependent reference ranges.2 One assay may underestimate values at the lower end of the range, while overestimating values at the higher end of a range.2 The differences in some cases may be small and not of clinical consequence unless values are being compared over time in the same animal. Ideally a veterinary laboratory that has a test validated for equine samples and can provide a laboratory specific reference range for horses should be used. For monitoring purposes, it is warranted to reuse the same laboratory, so that results can be compared over time. Contrary to previous advice, it is actually beneficial to test in the autumn months. Although normal horses will have slightly elevated ACTH concentrations in the autumn, PPID horses will have a markedly elevated ACTH concentration, thus making it easier to diagnose a horse in the early stages of disease in the autumn.3 Seasonal reference ranges should therefore be used, and care should be made when comparing subsequent test results performed at different times of the year.4,5 Seasonal reference ranges have recently been published for Perth and Townsville.6 Diurnal variations are not enough to interfere with a diagnosis of PPID, there sampling can be performed at any time of the day.7 In contrast to previous advice, there is no advantage of testing more than one sample from a single day.7 Testing should not be performed immediately after exercise, in acutely ill or severely painful animals