GSTM1 and colorectal carcinoma: It works in classical Bayesian fashion: from a prior risk of CRC of 1.3% in the symptomatic population, a result over the 10 μg Hb/g faeces threshold increased the risk to 7.0%, and a result below the threshold reduced it to 0.2%, which is approximately the whole population background risk, including those without symptoms.26 Furthermore, in five of the eight cancers with a result under the threshold, the patient’s GP still requested urgent investigation for possible CRC, probably because continuing symptoms allowed the GP to ‘overrule’ the negative test.