Norethisterone interference in testosterone assays.

BACKGROUND: This article describes three patients in whom measured serum testosterone concentrations were found to be artifactually high due to interference from norethisterone medication. This interference was investigated further by distributing samples containing norethisterone through an external quality assessment scheme. METHODS: Serum samples containing different concentrations of norethisterone were distributed to participants in the UK external quality assessment scheme (UK NEQAS) for female testosterone in order to assess the degree of interference from norethisterone in different commercially available immunoassay and liquid chromatography-tandem mass spectrometry methods for measurement of testosterone. RESULTS: The results have shown that apparent serum testosterone concentrations in excess of 5 nmol/L may be obtained using the Roche E170 Modular immunoassay method for samples collected from patients taking norethisterone medication and this interference can be reproduced by adding norethisterone to serum samples. CONCLUSIONS: Although the biggest interference is seen with the Roche system, there is a small effect in the Siemens ADVIA Centaur assay and some of the other immunoassays may also be affected to a much lesser extent. Norethisterone does not interfere in testosterone measurements obtained by liquid chromatography-tandem mass spectrometry.