Performance of immunochemical faecal occult blood test in colorectal cancer screening in average‐risk population according to positivity threshold and number of samples

Abstract

Immunochemical faecal occult blood tests (I-FOBT) detect more effectively advanced neoplasia than guaiac tests (G-FOBT). The study aim was to compare the performance of an I-FOBT whilst varying the positivity threshold and considering four analysis modalities: one sample was performed (MG₁), two samples were performed and at least one sample was positive (MG₂₊), both samples were positive (MG₂₊₊) or the mean of the two samples' log-transformed haemoglobin contents exceeded the cutoff (MG_2m). Screening for colorectal cancer using both G-FOBT and two samples' I-FOBT was performed by an average-risk population sample of 20,322 subjects. Among the 1,615 subjects with at least one positive test, 1,277 had a satisfactory colonoscopy result; 43 invasive cancers and 270 high-risk adenomas were detected. The I-FOBT was reinterpreted under each analysis modality (a random selection of one sample led to MG₁). For all modalities, increasing the positivity threshold decreased sensitivity and increased specificity. The relative ROC curves (in reference to G-FOBT) demonstrated similar performance for MG₁ and MG₂₊, and improved performance for MG_2m. MG₂₊₊ sensitivity was limited within the range of positivity thresholds evaluated. For any specificity, MG_2m provided the highest sensitivity. For any sensitivity, MG_2m provided the highest specificity. For any positivity rate, MG_2m provided both the highest sensitivity and specificity. This study suggests the replacement of MG₂₊ by MG₁ or, for even better performance, by MG_2m provided that two samples are performed with similar participation (which should be explored). The targeted positivity rate could then be achieved by choosing the positivity threshold. © 2009 UICC