Pituitary pars intermedia dysfunction (PPID) occurs commonly, affecting 20% of equids over 15 years of age and 30% of equids over 30. About one-third of PPID horses have concurrent insulin dysregulation, placing them at increased risk of endocrinopathic laminitis. Accurately diagnosing PPID involves measuring plasma adrenocorticotropic (ACTH) levels.
Relationship Between PPID and CLIP
“The current test for ACTH is a chemiluminescent immunoassay (iACTH), but this assay has cross-reactivity with corticotropin-like intermediate peptide (CLIP), both of which are cleavage products of proopiomelanocortin,” explained Kelsey Hart, DVM, PhD, DACVIM (LAIM), from the Department of Large Animal Medicine at University of Georgia’s College of Veterinary Medicine, during her presentationat the 2025 American Association of Equine Practitioners convention.
To further explain the relationship between PPID and CLIP, Hart said, “The key is that in PPID both ACTH and CLIP actually increase. In some horses it is just ACTH, in some horses it is just CLIP, and in some (many) it is both.”
Hart said we don’t understand this well because CLIP has not yet been measured outside research settings.
“For many years when we used the Immulite ‘ACTH’ assay to develop all our diagnostic criteria for PPID, we were basing those cutoff values on what was actually a measurement of ACTH and some CLIP,” she explained. “If we then moved to a stall-side assay that only measured ACTH and had no cross-reactivity with CLIP, then that assay would not agree well with the Immulite. That would be problematic because all our diagnostic criteria for PPID are based on the Immulite ‘ACTH’ value. In order to use the stall-side assay with these criteria, it needs to pick up both ACTH and CLIP.”
The current test for ACTH must also be sent to a laboratory, and those samples can be affected by handling that can be challenging to achieve in a field setting. Thus, a point-of-care (POC) assay would be beneficial for equine practitioners, especially in acute laminitis cases when diagnosing PPID emergently would be beneficial.
POC eACTH Assay
Hart described a commercially available POC that measures both ACTH and CLIP concurrently, combining them to produce a reading described as “eACTH.” This POC is a sandwich immunoassay using “bulk acoustic wave” technology.
“While data provided by the manufacturer of the POC eACTH assay demonstrates good performance based on response-concentration curves, this assay has not been examined across seasons,” she said. “And we know there is a lot of seasonal changes in ACTH and CLIP.”
The objective of Hart’s study was to compare the performance of this POC eACTH assay against the reference iACTH assay in 13 PPID and 15 non-PPID horses owned by the University of Georgia. Blood was collected in EDTA tubes six times a year and measured by both assays (i.e., eACTH in Hart’s laboratory and iACTH at Cornell University). In addition, intra- and inter-assay precision and linearity were also measured.
In total, Hart collected and compared 165 samples. Results showed:
- Good linearity (R2 = 0.991).
- Very good intra- and inter-assay coefficient of variation (8.47% and 14.1%, respectively).
- Good agreement between the two assays (r = 0.95, P<0.001) across the entire data set (all animals, all months).
- Low bias across the entire data set (17 pg/mL).
“The POC did read a little lower with the highest hormone concentrations, in samples with eACTH over 90 pg/mL,” Hart noted.
Bland-Altman plots for seasons showed a larger bias in the autumn (35.6 pg/mL in September) than non-autumn months (1.6 pg/mL in April). During the transition months (July and November), bias was intermediate.
“Considering just PPID animals of the entire data set and all months, a strong and significant correlation and good bias between the two assays was appreciated,” said Hart. “The bias month-to-month was larger in September compared with April, which was not surprising because we know that there are lots of ACTH and CLIP fluctuation and release of both hormones in the autumn months, especially in PPID horses.”
Final Thoughts
In conclusion, the eACTH POC assay had very good precision and linearity and excellent agreement with the reference iACTH method across seasons and across a range of ACTH concentrations in both healthy and PPID animals. There was slightly less agreement in August/September. Although the POC assay sometimes reads lower in samples with high ACTH concentrations, Hart said those ACTH values were nowhere near the diagnostic threshold for diagnosing PPID and would not affect interpretation of the test results.
“In sum, this assay is a reliable, rapid, and accurate POC assay than can benefit practitioners in the field setting,” Hart stated.
Medical coverage from the 2025 AAEP Convention is brought to you by American Regent Animal Health.
Related Reading
- Equine Stallside Testing: Elevating Patient Care
- New Point-of-Care ACTH Assay in Horses
- Impact of Transportation on ID Testing in Horses
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