POEMs Research Summaries
Clinical Question: How accurate are the new fecal DNA and fecal immunochemical tests as screening tests for colorectal cancer?
Fecal DNA is more sensitive but less specific than fecal immunochemical testing (FIT), and as a result has a higher false positive rate. The fecal DNA test is also more expensive than other noninvasive alternatives such as FIT. We do not know which test will be better at reducing mortality. (LOE = 2b)
Imperiale TF, Ransohoff DF, Itzkowitz SH, et al. Multitarget Stool DNA Testing for Colorectal-Cancer Screening. N Engl J Med 2014;370(14):1287-1297.
Study design: diagnostic test evaluation
Fecal DNA testing looks for abnormalities characteristic of the DNA in colorectal cancer (CRC), while fecal immunochemical testing (FIT) is an improved version of the older tests that detect fecal occult blood but only requires a single stool specimen. Adults aged 50 years to 85 years, at average risk for CRC, who were undergoing screening colonoscopy were invited to participate in the study. A total of 11,016 agreed, and underwent the required tests. Of that group, 689 were excluded because of an insufficient specimen for fecal DNA or a specimen that leaked in shipping (ick!), 304 others were excluded because of inadequate colonoscopy, and 34 had an insufficient sample for FIT. Of the final group of 9989 participants, 65 received a diagnosis of cancer. Fecal DNA testing was 92% sensitive and 87% specific, while FIT was 74% sensitive and 95% specific. Although this looks like a clear advantage for fecal DNA (60 of 65 cancers detected, compared with 48 of 65 for FIT), it is important to look a bit further. The lower specificity for fecal DNA meant that there were nearly 3 times as many false positive results that would have required a follow-up colonoscopy if fecal DNA was the sole screening test (1231, vs 472 for FIT). Using fecal DNA, there would have been 22 colonoscopies per cancer detected, compared with 11 using FIT. Also, fecal DNA testing requires the entire stool specimen, collected using a small bucket that hangs in the toilet, and costs approximately $400 to $800 (FIT costs approximately $3 to $40). Remember, because CRC takes several years to progress from adenoma to cancer, a FIT test could be performed annually at a much lower cost and likely detect many of the initially missed cancers in subsequent years.
Mark H. Ebell, MD, MS
University of Georgia