8-5 The ability to measure hormone levels based on a blood spot (like that used by diabetics for glucose monitoring) has several advantages over measurements based on a blood draw. First, blood spots allow for repeated measurements over the time course of minutes and hours.

Second, they can be collected with minimal training by a research assistant or the subject. Finally, they are easy to store in a variety of experimental conditions. The low levels of hormone in both blood spots and blood draws are currently measured by a technique called the radioimmunoassay (RIA), an assay based on binding of a radioactively labeled hormone to a specific antibody. Elizabeth Shirtcliff and colleagues‡

used a modification of a commercially available RIA to detect estradiol (the primary estrogen found in humans) in blood spots and compared?