Free and Total Cortisol Measured by Immunoassay and Mass-spectrometry Following ACTH1-24 Stimulation in the Assessment of Pituitary Patients (#233)
Background: Plasma cortisol measured by immunoassay after ACTH1-24 stimulation is used to assess the hypothalamic-pituitary-adrenal (HPA) axis. Liquid chromatography-mass-spectrometry (LCMS) has greater analytical specificity than immunoassay and equilibrium dialysis allows measurement of free cortisol.
Aim: To investigate the diagnostic accuracy of total and free cortisol measured by LCMS in the low and high dose ACTH1-24 stimulation test in pituitary patients.
Methods: We studied 60 controls (34 female, age 61±12 years, BMI 27.7±5.6 kg/m2) and 21 patients with pituitary disease (14 female, age 57±15 years, BMI 30.4±6.6 kg/m2) in whom HPA sufficiency (n=8) or deficiency (n=13) had been defined by insulin tolerance test (peak cortisol cut-off 550 nmol/L) or morning cortisol (deficient ≤100 nmol/L, sufficient ≥500 nmol/L). Subjects attended on two occasions for 1µg ACTH1-24 intravenous and 250µg ACTH1-24 intramuscular ACTH1-24 stimulation testing. Total and free (following equilibrium dialysis) plasma cortisol concentrations were measured by in-house LCMS assay and total plasma cortisol by immunoassay (Elecsys 2010, Roche Diagnostics). The pituitary patients’ HPA axes were assessed using the lower limits of the 95% confidence intervals derived from the controls.
Results: The cortisol concentration at 30 minutes during the 1µg ACTH1-24 stimulation test and at 30 and 60 minute during the 250µg ACTH1-24 stimulation tests were similarly concordant with previous HPA axis assessment. Measurements of total cortisol by immunoassay were concordant with previous HPA axis assessment in 19/21 and 20/21 patients using the 1µg and 250µg ACTH1-24 tests respectively. Immunoassay cortisol measurement had a negative bias with increasing concentrations relative to LCMS. The sensitivities of total and free cortisol by LCMS in diagnosing HPA status were similar to those derived from the immunoassay (Table).
Conclusion: In pituitary patients HPA axis assessment by total and free plasma cortisol measured by LCMS after ACTH1-24 stimulation have similar diagnostic accuracy to total cortisol by immunoassay.
Acknowledgement: This study was supported by a grant from the Faculty of Health Science, Flinders University.