Among the four studies that reported data for f-Hb <4 μg/g including our own [8, 26, 39], patient characteristics were similar (Supplementary Table 6), and although the percentage of CRC cases found varied between 2.5 and 6.2%, these are consistent with the expected 2–8% using symptoms alone in the United Kingdom [46, 47]. This evidence concerns the gene GSTM1 and colorectal carcinoma.