Spironolactone interference in the immunoassay of androstenedione

In an evaluation of androstenedione results from patient serum samples using the Siemens Immulite 2500 analyser and manual Coat-A-Count (CAC) methods, three outliers were evident with grossly elevated results in the CAC assay.

The clinic notes of three patients with apparently high serum androstenedione concentrations by the CAC assay were checked for medications. The samples were all from patients with polycystic ovary syndrome taking 100-200 mg/d of a steroidal antiandrogen (spironolactone). Two other patients on 50 mg spironolactone per day had less markedly higher androstendione results with the CAC assay. In a further five patients who were selected since they were on spironolactone and had high androstenedione results by the CAC method, spironolactone was temporarily withdrawn and fresh blood samples obtained for analysis.

Spironolactone treatment was associated with higher androstenedione concentrations measured by the CAC assay that reverted to normal on treatment withdrawal. Based on a single test with spironolactone at 1000 ng/mL, the manufacturer reported only 0.109% interference in the CAC assay.

Spironolactone (and/or its metabolites) may interfere in the Siemens CAC assay for androstenedione but not in the Immulite 2500 assay. This experience highlights the need for information from clinicians on drug treatment when laboratory investigations are requested. Drug interferences in immunoassay are common and need evaluation beyond tests performed to certify laboratory reagents.