Colorectal Cancer Screening

Introduction to Guaiac Fecal Occult Blood Test (gFOBT)

The National Comprehensive Cancer Network recommends that any positive gFOBT should be followed up with colonoscopy, as repeating the stool test or follow-up with non-colonoscopy tests is inappropriate 1, 2, 3
Recent consumption of red meat, certain vegetables with peroxidase activity, and vitamin C consumption in excess of 250mg can affect gFOBT results, potentially leading to false positives or negatives 1, 4, 2

The sensitivity of gFOBT for detecting colorectal cancer ranges from 37.1% to 79.4%, and specificity ranges from 85% to 99%, according to the American Cancer Society 4
Single-sample office-based testing is inadequate for gFOBT, and proper collection technique is essential, with tests performed on 3 successive stool specimens obtained at home while following dietary restrictions, as recommended by the American College of Gastroenterology and the National Comprehensive Cancer Network 4, 2, 5

Complete colonoscopy is the definitive next step after a positive gFOBT, as recommended by the National Comprehensive Cancer Network, the American Gastroenterological Association, and the American College of Gastroenterology 1, 2, 5, 3
Laboratory assessment, including complete blood count, iron studies (serum iron, total iron binding capacity, serum ferritin, and transferrin saturation), and liver function tests, should be considered in the investigation of a positive gFOBT result to assess for anemia, iron deficiency, and potential metastatic disease 1, 6
Microcytosis may reflect iron deficiency from chronic GI blood loss, while macrocytosis may be associated with vitamin B12 or folate deficiency, and abnormal white blood cell count or platelet count may reflect more generalized disturbances, as stated by the American Journal of Kidney Diseases 6
Carcinoembryonic antigen (CEA) should be ordered if colorectal cancer is confirmed, as part of a comprehensive diagnostic workup 6

Do not repeat the stool test if the initial test is positive - proceed directly to colonoscopy, as recommended by the American Gastroenterological Association and the National Comprehensive Cancer Network 2, 1
Do not attribute fecal occult blood solely to hemorrhoids without proper colonic evaluation, as stated by the American College of Gastroenterology 5
Single-sample in-office FOBT during digital rectal examination is not recommended due to very low sensitivity, as stated by the American College of Gastroenterology and alternative sources 7