Performance of the immunochemical fecal occult blood test in predicting lesions in the lower gastrointestinal tract

Abstract

Background: Previous studies have suggested that the immunochemical fecal occult blood test has superior specificity for detecting bleeding in the lower gastrointestinal tract even if bleeding occurs in the upper tract. We conducted a large population-based study involving asymptomatic adults in Taiwan, a population with prevalent upper gastrointestinal lesions, to confirm this claim.

Methods: We conducted a prospective cohort study involving asymptomatic people aged 18 years or more in Taiwan recruited to undergo an immunochemical fecal occult blood test, colonoscopy and esophagogastroduodenoscopy between August 2007 and July 2009. We compared the prevalence of lesions in the lower and upper gastrointestinal tracts between patients with positive and negative fecal test results. We also identified risk factors associated with a false-positive fecal test result.

Results: Of the 2796 participants, 397 (14.2%) had a positive fecal test result. The sensitivity of the test for predicting lesions in the lower gastrointestinal tract was 24.3%, the specificity 89.0%, the positive predictive value 41.3%, the negative predictive value 78.7%, the positive likelihood ratio 2.22, the negative likelihood ratio 0.85 and the accuracy 73.4%. The prevalence of lesions in the lower gastrointestinal tract was higher among those with a positive fecal test result than among those with a negative result (41.3% v. 21.3%, p < 0.001). The prevalence of lesions in the upper gastrointestinal tract did not differ significantly between the two groups (20.7% v. 17.5%, p = 0.12). Almost all of the participants found to have colon cancer (27/28, 96.4%) had a positive fecal test result; in contrast, none of the three found to have esophageal or gastric cancer had a positive fecal test result (p < 0.001). Among those with a negative finding on colonoscopy, the risk factors associated with a false-positive fecal test result were use of antiplatelet drugs (adjusted odds ratio [OR] 2.46, 95% confidence interval [CI] 1.21-4.98) and a low hemoglobin concentration (adjusted OR 2.65, 95% CI 1.62-4.33).

Interpretation: The immunochemical fecal occult blood test was specific for predicting lesions in the lower gastrointestinal tract. However, the test did not adequately predict lesions in the upper gastrointestinal tract.

Figure 1:

Study flow of asymptomatic participants undergoing immunochemical fecal occult blood test (iFOBT), colonoscopy and esophagogastroduodenoscopy (EGD).

Figure 2:

Risk factors associated with 233 false-positive results of the immunochemical fecal occult blood test among 2122 participants with a negative finding on colonoscopy. The multivariable model adjusted for all variables shown in the forest plot. An odds ratio greater than 1.0 indicates an increased risk of a false-positive result. CI = confidence interval. *Includes diabetes mellitus, hypertension and cardiovascular disease. †Normal hemoglobin concentration: < 120 g/L in women and < 130 g/L in men. ‡Low platelet count: <150 × 10⁹/L.