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BACKGROUND: Differentiation between subtle changes in low serum testosterone concentrations, common in women and children, is not possible with current commercially available assays. The objectives of the study were to develop a method based on stable isotope dilution-liquid chromatography-tandem mass spectrometry (ID-LC-MS/MS) with adequate sensitivity and specificity and to investigate the applicability of this assay in serum samples from pre- and postmenopausal women. METHODS: For 16 women, testosterone levels were measured in blood samples drawn two years before and after physiological menopause, and for eight women in samples drawn before and after bilateral oophorectomy. Testosterone was extracted from serum, derivatized and analysed on an LC-MS/MS. RESULTS: The developed ID-LC-MS/MS method allowed for specific and reproducible measurement of testosterone. Comparison with stable isotope dilution-gas chromatography coupled to mass spectrometry detection by Deming regression analysis gave a slope of 1.025 and an intercept of 0.055 nmol/L (r = 0.9998). A significant decrease was found in testosterone concentrations before and after bilateral oophorectomy (P = 0.02), whereas no significant difference was found before and after natural menopause (P = 0.4). CONCLUSIONS: The ID-LC-MS/MS assay measures serum testosterone with acceptable accuracy and is useful in female samples, supporting the conclusion that the postmenopausal ovary contributes to circulating testosterone. To our knowledge, our analytical method compares favourably to similar published methods in terms of sensitivity. The sensitivity and specificity of this method comply with the reference method for measurement of testosterone in serum samples of women, children and men suffering from hypogonadism and can also be used for men with testosterone in the reference range. [ABSTRACT FROM AUTHOR]