GSTM1 and colorectal carcinoma: Low f-Hb thresholds of <2, <7, <10 and <20 μg/g gave respective CRC risks of 0.1, 0.3, 0.3 and 0.4%, numbers needed to scope for one CRC of 871, 335, 300 and 249, and ‘false negative’ rates of 2.9, 11.4, 13.3 and 17.1%.