title: Effect of faecal occult blood positivity on detection rates and positive predictive value of CT colonography when screening for colorectal neoplasia.
abstract: OBJECTIVE
  To determine the detection rates and positive predictive value (PPV) of computed tomography (CT) colonography (CTC) according to the magnitude of faecal occult blood test (FOBt) positivity.
  
  
  METHODS
  Anonymised data from individuals undergoing CTC after a positive FOBt in the English Bowel Cancer Screening Programme were analysed. The detection of colorectal cancer (CRC), advanced neoplasia, and ≥ 6 mm polyps were stratified by the number of positive FOBt windows. The PPV was calculated by reference to subsequent endoscopy results. The influence of the FOBt result on detection rates was estimated with multilevel logistic regression. PPV, CRC stage, and location were compared across groups according to FOBt positivity.
  
  
  RESULTS
  Four thousand, six hundred and one individuals were included (mean = 66.7 years, 54.2% men). Detection rates of CRC and advanced neoplasia increased with greater numbers of positive FOBt windows (odds ratio [OR] for CRC = 1.41; 95% confidence interval [CI]: 1.31-1.52; OR for advanced neoplasia = 1.17; 95%CI: 1.12-1.23; both p < 0.0001). The PPV was significantly greater at higher FOBt levels (p = 0.020). The number of positive FOBt windows had no significant effect on stage (p = 0.30) or location (p = 0.20) of confirmed CRC.
  
  
  CONCLUSIONS
  The magnitude of FOBt positivity influences the PPV and detection rates when screening for colorectal neoplasia. CTC may be particularly useful for FOBt patients with few positive test windows.
authors.affiliation: Centre for Medical Imaging, Podium Level 2, University College Hospital, 235 Euston Rd, London NW1 2BU, UK. Electronic address: aaoplumb@doctors.net.uk.
authors.forename: A A
authors.initials: AA
authors.lastname: Plumb
authors.affiliation: Centre for Medical Imaging, Podium Level 2, University College Hospital, 235 Euston Rd, London NW1 2BU, UK.
authors.forename: S A
authors.initials: SA
authors.lastname: Taylor
authors.affiliation: Centre for Medical Imaging, Podium Level 2, University College Hospital, 235 Euston Rd, London NW1 2BU, UK.
authors.forename: S
authors.initials: S
authors.lastname: Halligan
mesh_terms: D000368:Aged; D023881:Colonography, Computed Tomographic / Q000379:methods*; D015179:Colorectal Neoplasms / Q000097:blood* / Q000175:diagnosis / Q000000981:diagnostic imaging*; D055088:Early Detection of Cancer / Q000379:methods; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009780:Occult Blood; D011237:Predictive Value of Tests; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity