Rosner explained that each successive assay method has made it easier to meet the growing need for estradiol testing, but all also have their own challenges. “The early radioimmunoassays involved extracting the biological fluid, followed by a chromatography step, and then the assay itself. They had good sensitivity and specificity at that time,” he said. “To get more efficient, direct immunoassays were developed which didn’t require extraction and chromatography steps. That led to the ability to automate, which drove the cost down considerably. However, the price of that automation was very often inaccuracy. Direct immunoassays work fine in some cases, but not very well in others. Estradiol and testosterone are examples of that.”
Arguably one of the most significant challenges for direct immunoassays is that antibodies in these tests cross-react with multiple forms of the hormone. According to the panel, estradiol circulates in blood as more than 100 conjugated and unconjugated metabolites. Furthermore, patients may have circulating estrogens from exogenous sources such as conjugated equine estrogens and nutritional supplements. Some of these forms circulate in high enough concentrations that even small cross-reactivities can cause “profoundly deranged results,” the panel wrote.
