Problems with direct testosterone immunoassays

Large differences were reported from measurements of the same serum sample with chemi-luminescent assays from different manufacturers [39,40]. Direct RIA techniques were not better [41]. In the low range (values of interest for castration control in patients with prostate cancer), which was close to range of female testosterone levels, direct assays gave results more than 20% different from the gold standard [41]. Abbot Architect assay was also reported to give consistently up to 20% higher results compared to standard in this range of values [39].

One of the reasons for variability is in the fact that antibodies are different among manufacturers, with different cross-reactivity profiles. All present direct chemiluminescent assays are matrix dependent, which was extensively studied by the British group [42]. It was confirmed there was significant cross-reactivity for example with dehydroepiandrosteronesul-phate (DHEA-S) [43]. The described issue is not only in urology regarding testosterone -also other areas of endocrinology where steroid hormones measurements are important, have reported and discussed similar issues [44,45]. College of American Pathologists proficiency testing revealed in 2008, highest mean compared to lowest mean for testosterone, to differ by factor 2.8 [46]. Differences for mass spectrometry assays were much lower, by factor 1.4.

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