Stool Testing

Stool testing, by one of three methods,  is considered an effective method of CRC screening.  Several large-scale studies have demonstrated that annual fecal occult blood testing (FOBT) using a guaiac-based test can decrease CRC mortality by 15-33%.  However, guaiac FOBT can be highly variable in sensitivity and specificity, depending on the brand or variant, collection technique, number of samples, whether samples are rehydrated, and how the results are interpreted.  Fecal immunochemical testing (FIT) has some technical advantages over guaiac FOBT, including the elimination of false negatives due to diet, although it has not been shown to have better overall accuracy than a sensitive guaiac FOBT (Hemoccult SENSA).  Stool DNA testing (sDNA) is not as well studied as FOBT and FIT, but is also considered an acceptable option in the current ACS guideline. (2)  

Guaiac FOBT

Guaiac FOBT has been shown, in randomized controlled trials, to be an effective screening method, with cancer detected at an earlier stage using these tests than in unscreened patients.  For guaiac FOBT, the ACS now recommends using only high-sensitivity tests that have been shown in the peer-reviewed medical literature to detect a majority of existing CRC in an asymptomatic population.  Lower-sensitivity tests are no longer recommended. (2)  The patient must complete three FOBT cards, following three consecutive bowel movements.  The accuracy of the test depends on the number of samples that are tested, since polyps and cancers may bleed intermittently.  A single FOBT card tested after digital rectal examination is not an acceptable substitute for the three-card FOBT that a patient completes at home, and using this practice for CRC screening should be abandoned. (2)  Guaiac FOBT should be repeated annually.

FOBT and Dietary Restrictions.  Because of potential interactions between dietary intake and guaiac-based tests, the manufacturer of a common sensitive guaiac FOBT card (Hemoccult SENSA, Beckman Coulter) recommends that patients abstain from NSAIDS for seven days, red meat for three days, and Vitamin C in doses greater than 250mg/day for three days before beginning the test and during the test period.  The concern is that NSAIDs may promote GI bleeding; peroxidase in red meat may cause a false-positive; and Vitamin C appears to increase false-negative results.  The current ACS guideline also highlights the potential for peroxidase-containing vegetables and certain fruits to cause false-positive results. (2)  Fruits and vegetables high in peroxidase include turnips, radishes, horseradish, broccoli, cauliflower, and cantaloupe.  Although recommendations for dietary restriction are routine, the precise influence of these dietary elements on test accuracy has not been established.

Some physicians feel that dietary restrictions make patients less likely to complete the FOBT cards. However, a meta-analysis of trials suggests that advice to perform modest dietary restriction during non-rehydrated FOBT does not affect the completion rate. (11) 

Fecal Immunochemical Testing (FIT).  A newer version of FOBT uses immunochemical testing for human globin, a specific protein in human hemoglobin.  FIT eliminates the need for dietary restrictions, does not require medication adjustments, and generally requires less handling of stool.  The American Cancer Society includes FIT in their screening recommendations, noting that these tests are more specific for human blood and, because globin is degraded by enzymes in the upper GI tract, are also more specific for lower GI bleeding. (2)

Comparisons of FIT and sensitive guaiac FOBT have not shown a clear pattern of superiority for one over the other in terms of detecting cancer, although the ACS guideline notes that specificity of FIT tended to be higher. (2)

FIT should be repeated annually.

Stool DNA Testing

Stool DNA (sDNA) tests look for genetic alterations that occur in colon adenomas and colon cancer.  sDNA testing has been evaluated in comparison with a low-sensitivity guaiac FOBT test in one large, prospective study of patients at average risk of colon cancer.  In this study, sDNA had better sensitivity than the guaiac test.  The currently offered version uses technology intended to further improve sensitivity; however, large studies on this version have not been done. 

Because sDNA testing involves a limited panel of genetic markers, it is likely that it misses a certain number of cancers which do not contain those genetic changes.  In addition, the significance of a positive DNA test with a negative follow-up colonoscopy is not known, and the appropriate interval between tests is uncertain. (2)  However, for patients who are not willing to do more traditional stool testing, the ACS guideline includes sDNA testing as a reasonable option.

Stool Tests and Patients at Increased Risk for CRC.  The above information regarding screening using stool testing applies to average risk, asymptomatic persons. Screening recommendations differ for persons at high risk of CRC. Anyone with symptoms of CRC should proceed directly to diagnostic testing.

Performing the tests

Guaiac FOBT

The following instructions are based on those for a common sensitive guaiac FOBT product.  Patients should refer to the manufacturer’s instructions for the test type and the specific version they are using.

For sensitive guaiac FOBT, the patient should prepare one FOBT card from each of three consecutive bowel movements.  There are two test slides per card. 

• Gather supplies (test cards and wooden applicator).  Each test card will have 2 windows in which a stool sample will be placed.  A complete fecal occult blood test consists of 3 cards, each with 2 stool samples, for a total of 6 samples.  This improves the accuracy of the test.

• If the cards are not already labeled with name, age, and address, write this information in the blanks on the front of each card. 
  
  
• Place a piece of plastic wrap on the toilet bowl (under the toilet seat) between seat and the toilet water, or use a clean container to catch stool.  Do not contaminate stool with urine or toilet tissue. 
  
  
• Use the wooden applicator to apply a thin film of stool onto one of the windows on the test card.
  
  
• Using the applicator again, take a specimen from a different place in the same stool sample and apply it to the other window on the test card
  
  
• Close the slots and write name and the date the sample was collected on the card. Store the card in a paper envelope; do not use a plastic bag or other sealed container that would prevent the slide from drying.
  
  
• Repeat the test on the next two bowel movements.

When the FOBT cards are returned, they should be developed immediately.  It is recommended that samples be developed within 7 days of collection.

Controversy has surrounded the issue of whether to rehydrate stool samples. The USPSTF states that rehydrating the samples raises sensitivity but lowers specificity, increasing the number of false positives. (15)  Guidelines outlined in a 2003 update from the American Gastroenterological Association recommend that the stool samples not be rehydrated. (5)

FIT

The following instructions are based on those for a common FIT product.  Patients should refer to the manufacturer’s instructions for the test type and the specific version they are using.

• Remove any automatic cleaning devices or bluing agents from the toilet, and flush twice.  If no cleaning agents are used, simply flush before your bowel movement.
  
  
• Do not place used toilet paper into the bowl.  
  
  
• Brush the surface of the stool with the supplied long-handled brush.
  
  
• Dab the brush onto the test card and close the card flap.
  
  
• Repeat the test on the next bowel movement, using a new card.

FIT samples are usually processed in a clinical laboratory.  Beckmann Coulter, manufacturer of Hemoccult ICT, an immunochemical test, states that samples for this FIT are stable at room temperature for 14 days.  However, samples should always be sent for analysis as soon as possible after the cards are complete.

sDNA

For sDNA testing, the entire stool specimen is used, and it must be shipped to the laboratory in a special container including an ice pack.  Patients should refer to the manufacturer’s instructions for the test type and the specific version they are using.

Risks of the Tests

Guaiac FOBT, FIT, and sDNA tests have minimal direct risk.  The primary risk is that a false positive will lead to unnecessary colonoscopy or other further tests.

Interpreting the Tests

Even with good sensitivity and specificity, the relatively low prevalence of colorectal cancer in the general population means that many people who have positive stool tests will not have cancer.  When positive, stool tests indicate the need for total colonic evaluation, which may reveal polyps or frank malignancy.  For FOBT and FIT, in which a complete test involves multiple samples, a positive test is defined as one or more positive samples.  The American Cancer Society recommends that patients with a positive stool test proceed directly to colonoscopy. Repeat stool testing is not recommended.  Persons too fragile to undergo complete diagnostic colonic evaluation should not be screened.

Guaiac FOBT and FIT should not be performed on persons who are likely to have misleading results, such as those with actively bleeding hemorrhoids.