Progesterone immunoassay cross-reactivity and similarity predictions. A. The plot shows the progesterone reference range for adult males and females (highlighted in yellow) in comparison to the predicted apparent progesterone concentrations produced on the Roche Elecsys Progesterone II assay by 5β-dihydroprogesterone, 17-hydroxyprogesterone (pediatric controls and patients with 21-hydroxylase deficiency), pregnanolone, allopregnanolone, medroxyprogesterone, 11-deoxycortisol (healthy controls, patients with 11β-hydroxylase deficiency, and following metyapon challenge), and exemestane. Table 4 contains the concentration ranges and percent cross-reactivity values from which the estimated apparent progesterone concentrations are derived. B. Two-dimensional similarity of compounds to progesterone is shown, sorted by degree of cross-reactivity in the Roche Progesterone II assay (horizontal line in each column indicates average similarity within that group). Similarity values vary from 0 to 1, with 1 being maximally similar. The compounds are subdivided into categories of strong cross-reactivity (5% or greater, black circles), weak cross-reactivity (0.5-4.9%, red squares), very weak cross-reactivity (0.05-0.49%, blue triangles), and no cross-reactivity (<0.05%, green diamonds) to the Roche Progesterone II assay (complete list of compounds and associated cross-reactivities and 2D similarities is in Additional file 1).