GSTM1 and Schnyder corneal dystrophy: A single quantitative f-Hb result, without any clinical information, could be sufficient to decide whom to refer for endoscopy and, because of the significant overlap of symptoms in those with and without SCD, could be the primary investigation performed.Please see related article: http://bmcmedicine.biomedcentral.com/articles/10.1186/s12916-016-0684-5 .