Evaluation of the sensitivity and specificity of basal plasma adrenocorticotrophic hormone concentration for diagnosing pituitary pars intermedia dysfunction in horses: A systematic review.
Authors: Tatum R C, McGowan C M, Ireland J L
Journal: Veterinary journal (London, England : 1997)
Summary
# Basal ACTH for PPID Diagnosis: Evidence Quality Matters Pituitary pars intermedia dysfunction (PPID) is a common endocrine disorder in older horses, and basal plasma ACTH measurement has become the standard diagnostic test, yet no systematic review had previously evaluated the robustness of evidence supporting this approach. Tatum and colleagues conducted a comprehensive literature search across three time points (2019–2021), ultimately including 11 peer-reviewed studies that reported sensitivity and specificity data for basal ACTH testing in horses suspected of PPID. The analysis found median sensitivity of 75.5% (range 36–100%) and median specificity of 95.2% (range 63.3–100%), suggesting the test performs well at identifying horses *without* PPID but is less reliable at confirming the condition. However, critical assessment using standardised quality appraisal tools (QUADAS-2 and STARD) revealed substantial methodological limitations across most included studies—including suboptimal study design, inappropriate reference populations, and reporting gaps—making it impossible to determine whether these diagnostic figures are truly representative. For practitioners, this means basal ACTH remains a valuable screening tool for ruling out PPID in horses with clinical signs, but the variable and sometimes modest sensitivity indicates that negative results should be interpreted cautiously; additional testing (such as the thyrotropin-releasing hormone stimulation test) may be warranted when clinical suspicion remains high.
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Practical Takeaways
- •A negative basal ACTH result is more reliable for excluding PPID than a positive result is for confirming it; consider further testing if clinical suspicion remains high despite normal ACTH
- •Awareness that published sensitivity estimates vary widely (36-100%) suggests test performance may differ based on lab reference ranges, population characteristics, or timing of sampling in your practice
- •Use basal ACTH as part of a broader diagnostic approach rather than a standalone test, particularly when clinical signs of PPID are present but ACTH is normal
Key Findings
- •Basal ACTH showed good median sensitivity of 75.5% (IQR 64.0-86.5%) for diagnosing PPID, but with wide range of 36.0-100% across studies
- •Basal ACTH demonstrated excellent median specificity of 95.2% (IQR 84.2-98.9%), indicating strong performance at ruling out PPID
- •11 of 415 identified publications met inclusion criteria; most studies had suboptimal design, study populations, or were subject to bias limiting evidence quality
- •Basal ACTH performed better at ruling out PPID than detecting it, suggesting its utility as a negative predictive test