Data plot and RCS regression for women aged 15 to 49 years. The plots show SF concentrations with median Hb (A; red line), and median eZnPP (B; red line), as well as RCS regressions with 5 knots for Hb (C; the vertical dashed line indicates the plateau point) and eZnPP (D; the vertical dashed line indicates the minimum point), in a healthy sample of US nonpregnant women aged 15 to 49 years (n = 4639) participating in NHANES III, 1988 to 1994 (results were published in a previous study¹⁰). Shaded areas between the dashed lines represent 95% CIs.