Spironolactone metabolite causes falsely increased progesterone in the Abbott Architect immunoassay

Immunoassays are important for routine clinical testing and medical diagnosis. However, they are limited by cross-reactivity especially at low analyte concentrations. There is a critical need to investigate compounds that can interfere with immunoassays. Herein, we describe the identification of canrenone, a spironolactone metabolite that falsely increases progesterone concentrations on the Abbott Architect i2000 Immunoassay. Serum samples and assay diluents were spiked with spironolactone or canrenone and progesterone concentrations were measured on the Architect i2000 and Immulite XPi immunoassay platforms. Blood samples from patients taking spironolactone were analyzed with liquid chromatography-tandem mass spectrometry to evaluate the intrinsic response of progesterone concentrations to the presence of canrenone. We measured approximately 10-fold higher progesterone concentrations on the Abbott Architect i2000 compared to reference immunoassay analyzers (Siemens Immulite XPi and Roche Cobas e601/602), suggesting an analytical error which is unique to the Architect i2000 antibody and/or assay conditions. By measuring serum progesterone after addition of spironolactone or canrenone to serum samples, we found that canrenone falsely increased progesterone on the Architect i2000 immunoassay. However, this interference was more pronounced at low serum progesterone concentrations. Moreover, a strong positive correlation was seen between canrenone and measured serum progesterone concentrations. Our investigations are important for individuals who require progesterone measurements using the Architect i2000 immunoassay, especially because it is unlikely for clinicians to order canrenone measurements alongside progesterone measurements for individuals taking spironolactone. Further research is needed to determine whether canrenone can influence progesterone measurements on other immunoassay systems.