Serum GDF15 is elevated in iron-deficient persons relative to iron-replete controls. Persons with iron level less than or equal to 8 μM (n = 21) and iron level more than or equal to 14 μM (n = 22) were selected from a serum bank available at the Hematology and Biochemistry Unit, John Radcliffe Hospital. Serum samples had been stored at 4°C for up to 5 days. (A) Serum concentration of GDF15 in iron-deficient and iron-replete groups was measured by ELISA. Data are presented in a box and whisker plot showing the median, upper value, lower value, upper quartile, and lower quartile for each group. The Mann-Whitney test was used to determine the significance of difference in GDF15 serum levels between the 2 groups (P < .001). (B) Correlation between iron blood levels and serum GDF15 as determined by Spearman correlation (n = 43). (C) Correlation between transferrin saturation and serum GDF15 as determined by Spearman correlation (n = 43). Plotted data are the means of duplicate ELISA measurements. Regression lines are plotted.