Clinically and temporally specific diagnostic thresholds for plasma ACTH in the horse.
Authors: Durham Andy E, Clarke Brenton R, Potier Julie F N, Hammarstrand Robert, Malone George L
Journal: Equine veterinary journal
Summary
# Editorial Summary: Temporally Specific ACTH Diagnostic Thresholds in Equine PPID Pituitary pars intermedia dysfunction (PPID) remains a frequent diagnosis in older horses, yet plasma ACTH concentration—the primary screening tool—has suffered from inflexible, year-round diagnostic thresholds that fail to account for the hormone's natural seasonal fluctuations. Durham and colleagues analysed ACTH data from over 75,000 horses to establish weekly diagnostic cutoffs throughout the year, revealing that thresholds remain relatively stable for most months but show substantial variation between mid-June and early December, peaking in late September and early October. Their statistical approach identified two distinct populations (high and low ACTH) and demonstrated that clinically categorised horses with high or low suspicion of PPID correlated well with these mathematically derived groups. For practitioners, this means that a single diagnostic threshold is inappropriate, particularly during the latter half of the year; instead, using week-specific or month-specific values—adjustable according to clinical presentation—offers improved diagnostic accuracy and allows tailored decision-making based on whether missing a case or over-diagnosing carries greater clinical risk in any given situation. This work provides a practical framework for more nuanced interpretation of ACTH results and highlights the need for laboratories to provide temporally adjusted reference ranges alongside raw values.
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Practical Takeaways
- •Do not rely on a single ACTH threshold year-round; use week-specific reference ranges, particularly between June and December when values fluctuate most
- •Interpret ACTH results in context of clinical signs rather than as a standalone diagnostic test—seasonal thresholds should complement, not replace, clinical assessment
- •Consider the clinical stakes when choosing your threshold: use lower thresholds (higher sensitivity) when you cannot afford to miss PPID, and higher thresholds (higher specificity) when overdiagnosis risks unnecessary treatment
Key Findings
- •Diagnostic thresholds for plasma ACTH vary significantly throughout the year, with greatest variability from mid-June to early December, peaking in late September and early October
- •Analysis of 75,892 horses identified two distinct populations (high-ACTH and low-ACTH) that correlated favorably with clinical suspicion groupings (n=4,036 high suspicion; n=3,022 low suspicion)
- •Temporally specific thresholds can be established for each week of the year to improve diagnostic accuracy when used alongside clinical judgment
- •Different sensitivity/specificity thresholds should be applied depending on clinical context and whether false-positive or false-negative diagnosis is least desirable