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To the Editor: Dr Cummings and colleagues¹ reported that measurement of serum estradiol levels in older postmenopausal women by sensitive radioimmunoassay (RIA) identified those at high risk of breast cancer, who may benefit most from raloxifene treatment. Although the authors claim to have used a highly sensitive estradiol RIA, they provide virtually no information about its reliability. Direct estradiol immunoassays are generally considered to be unreliable for quantifying very low estradiol levels because they lack sensitivity and specificity to measure these levels accurately.² This is because direct immunoassays for steroid hormones are generally carried out with commercial reagents (sold as kits) that are used in a fixed assay method without purification of the analyte. The methods typically require a small aliquot of sample, generally 0.1 to 0.2 mL. Thus, a highly sensitive standard curve is required to quantify reliably very low levels of an analyte in such small sample volumes. . . . [Full Text of this Article]

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