Abstract
Diagnosis of PPID in horses relies on measurement of plasma adrenocorticotrophic hormone (ACTH) concentrations. Because measured ACTH values vary between analytical platforms, and published diagnostic thresholds apply almost exclusively to the Immulite 1000 chemiluminescent assay (CLA), there is a need for validated, assay-specific thresholds for contemporary analysers. This study aimed to transfer and validate established Immulite 1000 diagnostic thresholds for plasma ACTH to three contemporary systems (Immulite 2000XPi, Tosoh AIA-900, and Tosoh AIA-360) using the Clinical Laboratory Standards Institute (CLSI) guidelines. Surplus clinical EDTA plasma samples were analysed across pairs of analysers to enable method comparison and reference interval transference. The Immulite 2000XPi showed close agreement with the Immulite 1000 (r=0.962, P<0.0001; bias -3.9 pg/mL (95% CI: -12.2-4.4)), enabling reliable linear transference of thresholds. Comparison of Immulite 2000XPi with the AIA-900 revealed moderate correlation and non-linear, concentration-dependent bias (r=0.770, P<0.0001; bias 30.8 pg/mL (95% CI: -110-171)), necessitating threshold-specific regression. The AIA-360 and AIA-900 showed excellent agreement (r=0.993, P<0.0001; bias 4.023 pg/mL (95% CI: -1.4-9.4)). Platform-specific low (90% sensitivity) and high (90% specificity) diagnostic thresholds were generated for four seasonal periods for all analysers. The proposed analyser-specific thresholds provide clinicians and laboratories with more validated guidance for interpreting plasma ACTH results across contemporary analysers, supporting more accurate diagnosis and management of PPID in horses.