Fecal Occult Blood Test (FOBT)
In FOBT testing, a person collects stool samples that are analyzed for the presence of small amounts of blood. Collection details vary somewhat for different tests, but typically involve collection of as many as three different specimens on 3 different days, with small amounts from one specimen smeared by a wooden stick on a card with two windows or otherwise placed in a specimen container.
The guaiac test identifies peroxidase-like activity that is characteristic of human and nonhuman hemoglobin. Thus, it will record blood from ingested meat, upper airway bleeding such as epistaxis, upper gastrointestinal (GI) bleeding, as well as colonic lesions.
Five controlled clinical trials have been completed or are in progress to evaluate the efficacy of screening utilizing the FOBT. The Swedish trial is a targeted study for individuals aged 60 to 64 years. The English trial selects candidates from lists of family practitioners. The Danish trial offers screening to a population aged 45 to 75 years who were randomly assigned to a control or study group.[3,4]
The Minnesota trial randomly assigned 46,551 men and women aged 50 to 80 years to one of three arms: colorectal cancer screening with guaiac-based, rehydrated FOBT every year (15,570), every 2 years (15,587), or control (15,394). This trial demonstrated that annual FOBT screening decreased mortality from colorectal cancer (CRC) by 33% after 18 years of follow-up (relative risk [RR], 0.67; 95% confidence interval [CI], 0.51-0.83, compared with the control group) and that biennial testing resulted in a 21% relative mortality reduction (RR, 0.79; 95% CI, 0.62-0.97). Some part of the reduction may have been attributed to chance detection of cancer by colonoscopies; rehydration of guaiac test slides greatly increased positivity and consequently increased the number of colonoscopies performed. Subsequent analyses by the Minnesota investigators using mathematical modeling suggested that for 75% to 84% of the patients, mortality reduction was achieved because of sensitive detection of CRCs by the test; chance detection played a modest role (16%-25% of the reduction). Nearly 85% of patients with a positive test underwent diagnostic procedures that included colonoscopy or double-contrast barium enema plus flexible sigmoidoscopy (FS). After 18 years of follow-up, the incidence of CRC was reduced by 20% in the annually screened arm and 17% in the biennially screened arm. With follow-up through 30 years, there was a sustained reduction in CRC mortality of 32% in the annually screened arm (RR, 0.68; 95% CI, 0.56-0.82) and 22% in the biennially screened arm (RR, 0.78; 95% CI, 0.65-0.93). There was no reduction in all-cause mortality in either screened arm (RR, 1.00; 95% CI, 0.99-1.01 for the annually screened arm; and RR, 0.99; 95% CI, 0.98-1.01 for the biennially screened arm). Important information that was not reported includes the treatment of CRC cases by stage by arm and the extent of CRC screening in each arm by FOBT, sigmoidoscopy, or colonoscopy after the completion of the trial protocol.[8,9]