Immunochemical testing of individuals positive for guaiac faecal occult blood test in a screening programme for colorectal cancer: an observational study

Background: Screening for colorectal cancer by use of guaiac-based faecal occult blood tests (FOBT) reduces disease-specific mortality. However, due to imperfect specificity, about half of individuals positive for guaiac FOBT are negative for neoplasia on colonoscopy. We aimed to assess whether the testing of individuals positive for guaiac FOBT in a screening programme for colorectal cancer by use of a sensitive immunochemical FOBT could select more appropriately those who should receive colonoscopy. Methods: We invited individuals who were guaiac FOBT positive in the second screening round of a pilot study in Scotland, UK, to give two samples, each from separate stools, for immunochemical FOBT while awaiting colonoscopy. Results were classed as: both samples negative (N/N), one sample negative and one positive (N/P), and both samples positive (P/P); data were assessed for sampling bias. We compared immunochemical findings with those from colonoscopy using odds ratios of positive samples (P/P) versus negative (N/N and N/P). Sensitivity, specificity, and positive and negative likelihood ratios for cancer, and for cancer and high-risk adenomatous polyps were also calculated. Findings: 1486 participants were invited and 801 (54%) sets of duplicate samples were returned. We found no evidence of sampling bias with regard to sex, age, or degree of positivity on guaiac FOBT. Of 800 sets returned with consent and analysed, 173 (22%) were N/N, 129 (16%) were N/P, and 498 (62%) were P/P. ? test showed a highly significant positive correlation between degree of positivity on guaiac FOBT and on immunochemical FOBT (p