Introduction: Prednisolone is well known to interfere in routine immunoassays for cortisol with uncertainty surrounding the degree of this interference. Methods using tandem mass spectrometry technologies offer more specificity, however there have been some reports of prednisolone interference in mass spectrometry assays. We aimed to characterise the extent of prednisolone interference in both an automated immunoassay and a tandem mass spectrometry method (TMS).
Methods: Sera were spiked with prednisolone to a final concentration of 200 μg/l (555 nmol/l), 500 μg/l (1388 nmol/l) or 1000 μg/l (2776 nmol/l) n=20 at each concentration. These samples were then analysed using a Roche chemiluminescent immunoassay and a TMS assay. The calibration of the tandem mass spectrometry assay had been checked using a reference material. A sample containing 1390 μg/l (3856 nmol/l) prednisolone was prepared and analysed using the TMS assay to quantify any interference.
Results: Prednisolone was found to have a significant interference in the immunoassay at clinical concentrations. The degree of this interference increased disproportionately with the amount of prednisolone added. In the TMS assay, a sample containing 1390 μg/l (3856 nmol/l) prednisolone was not found to cause a peak above the lower limit of the assay. Comparing the spiked and unspiked samples at all concentrations of prednisolone showed no significant difference in the TMS assay.
Discussion: Despite previous reports of prednisolone interfering in mass spectrometry assays for cortisol, this was not found to be the case in our assay. This means that this mass spectrometry assay can give reliable cortisol results on people on prednisolone therapy.