Using CT colonography as a triage technique after a positive faecal occult blood test in colorectal cancer screening

Abstract

Objective: The purpose of this study was to evaluate the effectiveness of CT colonography (CTC) as a triage technique in faecal occult blood test (FOBT)-positive screening participants.

Methods: Consecutive guaiac (G-FOBT) and immunochemical (I-FOBT) FOBT-positive patients scheduled for colonoscopy underwent CTC with iodine tagging bowel preparation. Each CTC was read independently by two experienced observers. Per patient sensitivity, specificity and positive and negative predictive values (PPV and NPV) were calculated based on double reading with different CTC cut-off lesion sizes using segmental unblinded colonoscopy as the reference standard. The acceptability of the technique to patients was evaluated with questionnaires.

Results: 302 FOBT-positive patients were included (54 G-FOBT and 248 I-FOBT). 22 FOBT-positive patients (7%) had a colorectal carcinoma and 211 (70%) had a lesion >or=6 mm. Participants considered colonoscopy more burdensome than CTC (p<0.05). Using a 6 mm CTC size cut-off, per patient sensitivity for CTC was 91% (95% CI 85% to 91%) and specificity was 69% (95% CI 60% to 89%) for the detection of colonoscopy lesions >or=6 mm. The PPV of CTC was 87% (95% CI 80% to 93%) and NPV 77% (95% CI 69% to 85%). Using CTC as a triage technique in 100 FOBT-positive patients would mean that colonoscopy could be prevented in 28 patients while missing >or=10 mm lesions in 2 patients.

Conclusion: CTC with limited bowel preparation has reasonable predictive values in an FOBT-positive population and a higher acceptability to patients than colonoscopy. However, due to the high prevalence of clinically relevant lesions in FOBT-positive patients, CTC is unlikely to be an efficient triage technique in a first round FOBT population screening programme.

Figure 1

Flowchart of faecal occult blood test (FOBT)-positive participants. CTC, CT colonography; NPV, negative predictive value; PPV, positive predictive value.

Figure 2

Plot of the positive predictive value (PPV) versus the negative predictive value (NPV) when using different cut-off sizes for CT colonography (CTC) for detection of true colonoscopy lesions of ⩾10 mm and ⩾6 mm. The curve shows a plot of PPV versus 1–NPV. Results for detection of patients with lesions on colonoscopy of ⩾10 mm, for cut-off sizes for CTC lesions of ⩾8, 9, 9.5, 10, 10.5, 11 and 12 mm are shown. Results for detection of lesions of ⩾6 mm are shown for CTC cut-off sizes of 4, 5, 5.5, 6, 6.5, 7 and 8 mm.

Figure 3

(A) Degree of burden for both examinations overall. Participants found the colonoscopy examination significantly more burdensome than the colonoscopy preparation. (B) Degree of burden from CT colonography and colonoscopy bowel preparations. No significant difference was found between the degree of burden from the colonoscopy bowel preparation and the CT colonography bowel preparation.