The accurate measurement of cortisol by immunoassay is compromised by the potential for cross-reactivity of reagent antibodies with structurally-related steroid compounds present in patient serum. This susceptibility is potentiated when normal steroid metabolism is altered pharmacologically by anti-steroidogenic drugs. This class of drug is utilised in the management of Cushings syndrome to moderate cortisol production. To investigate the effect of the 11β-hydroxylase inhibitor metyrapone on serum cortisol assay, a comparison of measurement by immunoassay versus liquid chromatographytandem mass spectrometry (LCMS/MS) as gold standard was conducted.
Cortisol was measured in serum from three patient groups; i) patients receiving metyrapone therapy (n=112), ii) control group of patients diagnosed with Cushings syndrome currently receiving no treatment (n=32) and iii) control group of normal patients with no known adrenal pathology and not receiving medication known to interfere in the immunoassay of cortisol (n=79).
Bland-Altman plots showed good agreement between methods for the normal control group (bias=+2.5% (4.4 nmol/l)) and Cushings control group (bias=+1.3% (3.5 nmol/l)). This was in contrast to the metyrapone therapy group (bias=−23% (−59 nmol/l)). Passing-Bablok linear regression revealed bias observations were constant in nature with minimal contribution from proportional error. Pearson correlation coefficients for the three patients groups were ρ=0.39*, 0.17 and 0.36* for the normal control, Cushings control and metyrapone therapy groups, respectively (*indicates Pearson correlation coefficient significant at P<0.05). LCMS/MS results were positively correlated with the difference. Further interrogation of metyrapone group data showed the degree of difference between LCMS/MS versus immunoassay positively correlated with dose. This trend was also noted for serum 11-deoxycortisol concentration with dose.
In conclusion, these data show that the liability of immunoassay measurement of serum cortisol to interference in patients receiving metyrapone may lead to erroneous clinical decisions concerning dose titration.