Fecal immunochemical testing has lower sensitivity for advanced neoplasms of earlier stage, nonpolypoid morphology, and proximal location.
The finding "highlights the importance of future studies exploring whether this limitation really leads to less protection against proximal colon cancer," wrote Dr. Han-Mo Chiu in the July issue of Clinical Gastroenterology and Hepatology.
In the study, which was also published online Jan. 31, Dr. Chiu of the National Taiwan University Hospital, in Taipei, looked at 18,296 asymptomatic, prospectively enrolled, consecutive patients aged 50 years or older who underwent screening colonoscopy at the Health Management Center at the University Hospital between September 2005 and September 2010.
All participants collected their own fecal samples at home using the fecal immunochemical test (FIT), which detects human hemoglobin in the feces, 1 day prior to colonoscopy.
Overall, 59.2% of patients were male, and the mean age was 59.8 years.
The researchers found that the FIT was positive in 1,330 subjects (7.3%).
On colonoscopy, however, the researchers revealed nonadvanced adenomas in 3,385 patients (18.5%), advanced adenomas in 632 (3.5%), cancer in 28 (0.15%), and invasive cancer in 23 (0.13%) of the subjects.
That amounted to a sensitivity of FIT for nonadvanced adenomas, advanced adenomas, and cancer of 10.6%, 28.0%, and 78.6%, respectively, wrote the authors.
Looking at neoplasms classified according to World Health Organization criteria, the FIT also showed its sensitivity to be highly stage dependent. For example, that number was 28.0% for advanced adenomas, but climbed to 66.7% for carcinoma in situ plus T1 cancers, and 100% for T2 to T4 cancers (P for trend less than .001).
The location of the lesion also affected the reliability of the test. Excluding patients who were found to have both proximal and distal lesions, Dr. Chiu calculated that the sensitivity of FIT was significantly lower for proximal lesions than for overall advanced neoplasms (proximal vs. distal, 24.1% vs. 34.3%; P less than .013).
Finally, the researchers looked at the reliability of FIT in relation to the lesion morphology.
In this case, they determined that FIT sensitivity also was significantly lower for proximal or distal nonpolypoid advanced neoplasms, compared with polypoid lesions at a corresponding location (P less than .001 for both).
Regarding this last finding, "It is still not well understood why these lesions are less sensitive to detection by the FIT," wrote Dr. Chiu.
He added: "We speculated that their smaller surface areas in contact with feces, the sparser vasculature in the mucosa, and the more advanced degree of hemoglobin degradation during bowel passage for proximally located lesions provide partial explanations, and this issue is worth ... further investigation."
Nevertheless, early detection of these lesions, which "carry a higher risk for malignant transformation and invasiveness at a relatively smaller size when compared with their polypoid counterparts ... may help prevent interval cancer and improve the effectiveness of screening programs."
The authors disclosed no individual conflicts of interest. They reported that this study was partially supported by a research grant from the Department of Health of Taiwan.