Clinical implications of using adrenocorticotropic hormone diagnostic cutoffs or reference intervals to diagnose pituitary pars intermedia dysfunction in mature horses.
Authors: Horn Remona, Stewart Allison J, Jackson Karen V, Dryburgh Elizabeth L, Medina-Torres Carlos E, Bertin François-René
Journal: Journal of veterinary internal medicine
Summary
# Editorial Summary: ACTH Testing for PPID Diagnosis Pituitary pars intermedia dysfunction (PPID) diagnosis remains challenging in equine practice because ACTH concentrations fluctuate considerably with season and individual factors including age, sex, and body condition score. This prospective study of 106 mature horses (34 with confirmed PPID, 72 controls) compared two interpretive approaches—diagnostic cutoff values (DCOVs) determined monthly via receiver operating characteristic analysis, and reference intervals (RIs) derived using robust statistical methods—alongside baseline and thyrotropin-releasing hormone (TRH) stimulation testing. Baseline ACTH showed strong diagnostic accuracy (0.91), but the choice of interpretation method significantly altered clinical utility: DCOVs increased sensitivity from 61% to 87%, making them better for detecting mild cases, whilst RIs increased specificity from 85% to 98%, minimising false positives and unnecessary treatment. TRH stimulation further improved overall accuracy to 0.97, substantially strengthening diagnosis when baseline results are equivocal. For practitioners, this suggests that interpreting ACTH results should account for the clinical context—using DCOVs when case identification is the priority, RIs when avoiding overtreatment matters, and always considering TRH stimulation testing to improve diagnostic confidence, particularly in borderline presentations or early disease.
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Practical Takeaways
- •Using diagnostic cutoff values helps catch mild PPID cases but increases false positives; using reference intervals reduces unnecessary treatments but may miss early disease—choose based on clinical context
- •ACTH results must be interpreted considering the month of testing, horse age, sex, and body condition score due to significant physiological variation throughout the year
- •TRH stimulation testing provides the most reliable diagnosis of PPID and should be considered the preferred diagnostic approach over baseline ACTH alone
Key Findings
- •Baseline ACTH concentrations showed significant monthly variation (P<0.001) with interactions involving age, sex, and body condition score
- •Diagnostic cutoff values increased test sensitivity from 0.61±0.21 to 0.87±0.05 (P=0.002) for detecting PPID
- •Reference intervals increased test specificity from 0.85±0.12 to 0.98±0.01 (P=0.01), reducing false positives
- •TRH stimulation test improved overall diagnostic accuracy from 0.91±0.06 to 0.97±0.03 (P=0.004)