Free and total plasma cortisol measured by immunoassay and mass spectrometry following ACTH₁₋₂₄ stimulation in the assessment of pituitary patients.

CONTEXT: Measurement of plasma cortisol by immunoassay after ACTH₁₋₂₄ stimulation is used to assess the hypothalamic-pituitary-adrenal (HPA) axis. Liquid chromatography-tandem mass spectrometry (LCMS) has greater analytical specificity than immunoassay and equilibrium dialysis allows measurement of free plasma cortisol.
OBJECTIVE: We investigated the use of measuring total and free plasma cortisol by LCMS and total cortisol by immunoassay during an ACTH₁₋₂₄ stimulation test to define HPA status in pituitary patients. DESIGN AND
SETTING: This was a case control study conducted in a clinical research facility. PARTICIPANTS: We studied 60 controls and 21 patients with pituitary disease in whom HPA sufficiency (n = 8) or deficiency (n = 13) had been previously defined. INTERVENTION: Participants underwent 1 μg ACTH(1-24) intravenous and 250 μg ACTH₁₋₂₄ intramuscular ACTH₁₋₂₄ stimulation tests. MAIN OUTCOME MEASURES: Concordance of ACTH₁₋₂₄-stimulated total and free plasma cortisol with previous HPA assessment.
RESULTS: Total cortisol was 12% lower when measured by immunoassay than by LCMS. Female sex and older age were positively correlated with ACTH₁₋₂₄-stimulated total and free cortisol, respectively. Measurements of total cortisol by immunoassay and LCMS and free cortisol 30 minutes after 1 μg and 30 and 60 minutes after 250 μg ACTH₁₋₂₄ were concordant with previous HPA axis assessment in most pituitary patients. However, free cortisol had greater separation from the diagnostic cutoff than total cortisol.
CONCLUSIONS: Categorization of HPA status by immunoassay and LCMS after ACTH₁₋₂₄ stimulation was concordant with previous assessment in most pituitary patients. Free cortisol may have greater clinical use in patients near the diagnostic threshold.

RCT Entities:   Population Interventions Outcomes