Concentration curve of serum ferritin against hemoglobin, and soluble transferrin receptor in a sample of women who are blood donors. (A) Restricted cubic spline (RCS) regression fit with 5 knots of serum ferritin against Hb; vertical line denotes SF threshold calculated at the first plateau identified by differential equation. (B) RCS fit with 5 knots of serum ferritin against sTfR; vertical line denotes SF threshold calculated at the first minimum identified by differential equations in a healthy sample of US nonpregnant women aged 20 to 49 years from REDS-RISE.⁶