Clinical diagnostic accuracy of faecal occult blood test for anal diseases

To investigate the diagnostic accuracy of faecal occult blood test (FOBT) for anal diseases and to evaluate the relationship of this disorder to the results of FOBT.

In a hospital-based retrospective study, FOBT using faecal samples collected on two consecutive days was performed on patients with anal disease, such as internal as well as external haemorrhoids and anal fissure, patients with colorectal cancer and healthy subjects to evaluate the diagnostic accuracy of FOBT. In a screening-programme-based cross-sectional study, the subjects who underwent colorectal cancer screening by FOBT using the 2-day method were divided into two groups according to the results of a questionnaire on anal diseases; the positive rate of FOBT as well as the predictive value for colorectal cancer were determined in the two groups.

Among 300 cases, including 100 anal diseases, 100 colorectal cancers and 100 healthy subjects, the test was positive in 12 anal diseases, 82 colorectal cancers and six healthy subjects. The sensitivity to anal diseases and colorectal cancer was calculated to be 12% and 82%, respectively, and the specificity was 95%. A significant difference was noted in the sensitivity between anal diseases and colorectal cancer (P < 0.001). Among 21,724 subjects, who underwent colorectal cancer screening, 2395 subjects were positive for anal diseases, and 19,329 subjects were negative. The positive rate of FOBT was 7.2 for the anal diseases group and 6.8 for the non-anal diseases group, and the predictive value for colorectal cancer was 2.9 and 3.0, indicating no substantial difference between the two groups.

These results reveal that FOBT is inferior for the diagnosis of anal diseases, and anal diseases have little relationship to the results of FOBT.