Clinical implications of using adrenocorticotropic hormone diagnostic cutoffs or reference intervals to diagnose pituitary pars intermedia dysfunction in mature horses.

BACKGROUND: Diagnosis of pituitary pars intermedia dysfunction (PPID) is problematic because of large variations in ACTH concentrations. HYPOTHESIS/OBJECTIVES: Compare the test characteristics of baseline and post-thyrotropin-releasing hormone (TRH) stimulation plasma ACTH concentrations in horses using diagnostic cutoff values (DCOVs) and reference intervals (RIs) and determine the clinical consequences of using each method. ANIMALS: One hundred six mature horses: 72 control cases and 34 PPID cases. METHODS: Prospective case-controlled study. Horses underwent monthly TRH stimulation tests. Diagnostic cutoff values were determined monthly by receiver operating characteristic curves using the Youden index. Reference intervals were determined monthly by a robust method. For each case age, sex and body condition score (BCS) were recorded. RESULTS: Baseline ACTH concentrations varied by month (P < .001) with significant "month × age" (P = .003), "month × sex" (P = .003), and "month × BCS" (P = .007) effects. Baseline ACTH concentrations were accurate to diagnose PPID (0.91 ± 0.06) with DCOVs increasing the test sensitivity (0.61 ± 0.21 to 0.87 ± 0.05, P = .002) and RI increasing test specificity (0.85 ± 0.12 to 0.98 ± 0.01, P = .01). Thyrotropin-releasing hormone stimulation improved test accuracy (0.91 ± 0.06 to 0.97 ± 0.03, P = .004). CONCLUSIONS AND CLINICAL IMPORTANCE: ACTH concentrations follow a circannual rhythm and vary with physiological factors. As using DCOVs increases the ability to detect mild cases and using RI decreases the risk of unnecessary treatments, ACTH concentrations should be interpreted within a specific clinical context. The TRH stimulation test improves the diagnosis of PPID.

Immunoreactive insulin stability in horses at risk of insulin dysregulation.

BACKGROUND: Diseases associated with insulin dysregulation (ID), such as equine metabolic syndrome and pituitary pars intermedia dysfunction, are of interest to practitioners because of their association with laminitis. Accurate insulin concentration assessment is critical in diagnosing and managing these diseases. HYPOTHESIS/OBJECTIVES: To determine the effect of time, temperature, and collection tube type on insulin concentrations in horses at risk of ID. ANIMALS: Eight adult horses with body condition score >6/9. METHODS: In this prospective study, subjects underwent an infeed oral glucose test 2 hours before blood collection. Blood samples were divided into ethylenediaminetetraacetic acid, heparinized, or serum tubes and stored at 4 or 20°C. Tubes were centrifuged and analyzed for insulin by a chemiluminescent assay over 8 days. Changes in insulin concentrations were compared with a linear mixed effects model. RESULTS: An overall effect of time, tube type and temperature was identified (P = .01, P = 0.001, and P = 0.001, respectively). Serum and heparinized samples had similar concentrations for 3 days at 20°C and 8 days at 4°C; however, after 3 days at 20°C, heparinized samples had significantly higher insulin concentrations (P = .004, P = .03, and P = .03 on consecutive days). Ethylenediaminetetraacetic acid samples had significantly lower insulin concentrations regardless of time and temperature (P = .001 for all comparisons). CONCLUSIONS AND CLINICAL IMPORTANCE: These results suggest an ideal protocol to determine insulin concentrations involves using serum or heparinized samples with analysis occurring within 3 days at 20°C or 8 days at 4°C.

Evaluation of combined testing to simultaneously diagnose pituitary pars intermedia dysfunction and insulin dysregulation in horses.

BACKGROUND: The thyrotropin-releasing hormone (TRH) stimulation test and the 2-step insulin sensitivity test are commonly used methods to diagnose, respectively, pituitary pars intermedia dysfunction (PPID) and insulin dysregulation (ID). OBJECTIVES: To investigate the diagnostic value of combining the TRH stimulation test and the 2-step insulin sensitivity test to diagnose PPID and ID simultaneously. ANIMALS: Twenty-seven adult horses, 10 control horses without PPID or ID, 5 horses with PPID only, 5 horses with ID only, and 7 horses with PPID and ID. METHODS: Randomized prospective study. Horses underwent a TRH stimulation test alone, a 2-step insulin sensitivity test alone, and combined testing with simultaneous TRH and insulin injection in the same syringe. Data were compared by 2-way repeated measures analysis of variance and 2 1-sided tests to demonstrate equivalence. Bland-Altman plots were generated to visualize agreement between combined and independent testing. RESULTS: The effect of combined testing on plasma adrenocorticotropic hormone, blood glucose concentration, or percentage decrease in blood glucose concentration was not significantly different from the effect obtained with independent testing. One control horse appeared falsely positive for PPID, 2 PPID-only horses appeared falsely positive for ID, and 1 PPID and ID horse appeared falsely negative for ID when tests were performed simultaneously. Bland-Altman plots supported the agreement between combined and independent testing. CONCLUSIONS AND CLINICAL IMPORTANCE: Combining the TRH stimulation test and the 2-step insulin sensitivity test appears to be a useful diagnostic tool for equine practitioners in the field, allowing testing of a horse for both PPID and ID simultaneously.