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Am Fam Physician. 2000;61(6):1834-1837

The American Cancer Society recommends annual fecal occult blood testing (FOBT) of stool samples in patients 50 years of age or older. To decrease the number of false-positive results, patients are placed on dietary restriction and are asked to avoid using nonsteroidal anti-inflammatory drugs and aspirin for one week before testing. Despite these recommendations, stool samples for occult blood are often obtained at the time of digital rectal examination. Asymptomatic persons with a positive FOBT at the time of digital examination are often referred for colonoscopy. Bini and associates compared the diagnostic yield of colonoscopy in asymptomatic patients with a positive FOBT obtained by digital examination with that obtained by spontaneously passed stool (SPS) sampling.

Medical records were obtained from 672 consecutive patients more than 50 years of age who were at average risk for colorectal cancer and had a positive FOBT result. The FOBT was performed using standard kits without rehydration. The authors also studied the cost of identifying a source of occult gastrointestinal bleeding by colonoscopy in the two groups of patients.

A colonic source of occult gastrointestinal bleeding was identified in 145 patients. The predictive value of a positive FOBT was no different in those who were tested by digital examination (22 percent) and those whose occult bleeding was detected by SPS (21.3 percent). The number of patients with neoplastic lesions, adenomas, adenomas 1 cm or more in diameter and adenocarcinoma was similar in both groups. The findings of colonoscopic examinations were normal in 249 patients. There were no complications from colonoscopy in either group.

The average cost of performing colonoscopy was $885.12 per patient, with no difference in cost between the digital examination and the SPS groups. The cost of detecting an adenoma 1 cm or larger in diameter was slightly higher in the digital examination group, while the cost per adenocarcinoma found was slightly higher in the SPS group. The differences were not significant.

This is the first known study to compare the two methods of fecal occult blood screening in a group of asymptomatic patients at average risk for colorectal cancer. The low predictive value of a positive FOBT result is a well-described limitation of screening for colorectal cancer with guaiac-based tests. It is widely thought that testing for fecal occult blood at the time of digital examination increases the number of false-positive test results, thereby increasing the cost per cancer detected and the number of endoscopic complications. The study demonstrated that the cost per large adenoma and the cost per cancer detected was unaffected by the method of stool collection.

The authors conclude that FOBT of stool samples obtained by digital examination does not increase the number of false-positive results or the cost per neoplasm detected in asymptomatic patients. The authors recommend that a positive FOBT be evaluated by colonoscopy.

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