Evaluation of basal plasma ACTH levels for the diagnosis of PPID

This was a very large review of the recent published literature on diagnosing PPID using basal ACTH levels. Of 425 pieces 11 were considered worth inclusion in the review. Originally ACTH was not considered reliable until season concluded that the papers showed a 75% sensitivity and a 95% specificity on diagnosing PPID. What this means is if you get a negative (does not support the diagnosis of PPID) result there is a 25% chance your horse might still have PPID. However, if you get a positive (supports the diagnosis of PPID) result there is only a 5% chance he does not actually have PPID.

In summary, positive for PPID are very reliable, but negative for PPID, while supportive that your horse does not have PPID, it does not rule out the disease. There is much more on the diagnosis in the article above.
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Evaluation of the sensitivity and specificity of basal plasma adrenocorticotrophic hormone concentration for diagnosing pituitary pars intermedia dysfunction in horses: A systematic review

Vet J. 2021 May 21;105695.
Authors
R C Tatum 1 , C M McGowan 1 , J L Ireland 2
Affiliations

1 Institute of Infection, Veterinary and Ecological Sciences, Faculty of Health and Life Sciences, The University of Liverpool, Leahurst Campus, Neston, Cheshire, CH64 7TE, UK.
2 Institute of Infection, Veterinary and Ecological Sciences, Faculty of Health and Life Sciences, The University of Liverpool, Leahurst Campus, Neston, Cheshire, CH64 7TE, UK. Electronic address: Joanne.Ireland@liverpool.ac.uk.

Abstract

Measurement of basal adrenocorticotrophic hormone (ACTH) is currently used to diagnose pituitary pars intermedia dysfunction (PPID) in horses, yet a systematic review of the evidence for its use has not been undertaken. This study aimed to systematically review evidence regarding the sensitivity and specificity of the basal ACTH diagnostic test. Electronic databases were systematically searched in January 2019, September 2020 and January 2021, for English language publications published prior to these dates. Screening, data extraction and quality assessment of publications was undertaken by the authors using predefined criteria. Study design, methodology and information reported in included studies were assessed using Standards for Reporting of Diagnostic Accuracy (STARD) checklists. Risk of bias and applicability were appraised using the Quality Assessment tool for Diagnostic Accuracy Studies (QUADAS-2) quality assessment tool. Due to identified biases and marked between-study variations, meta-analysis was not undertaken. After removal of duplicates, 415 publications were identified, of which 25 were evaluated in full, with 11 of these meeting inclusion criteria. In most studies, basal ACTH was reported to have good sensitivity (overall median 75.5%; interquartile range [IQR], 64.0-86.5%; range, 36.0-100%) and excellent specificity (overall median, 95.2%; IQR, 84.2-98.9%; range, 63.3-100%). However, QUADAS-2 and STARD assessment highlighted that studies did not utilise optimal study design and/or study populations for the evaluation of a diagnostic test and the majority were subject to bias, or provided insufficient information to fully assess possible biases. Based on this review, basal ACTH performed better at ruling out PPID than detecting it.

Keywords: ACTH; Cushing’s disease; Diagnostic accuracy; Equine; PPID.