Almost six percent of Medicare patients needed surgery for colon cancer within three years after a negative colonoscopy.
A sample of five percent of Medicare enrollees identified 1,567 patients with colon cancer. Of those 89 or 5.7 percent had had a negative colonoscopy more than six months but less than three years previously. All 89 were of average risk for colorectal cancer.
Patients with high risk factors for colon cancer were not part of the study, including those with a history of colon cancer, inflammatory bowel disease, or family cancer syndromes.
Colonoscopies done in an office setting had the highest rate of missed polyps or cancers (15.7 percent), followed by those in an ambulatory center (7.8 percent). Hospital colonoscopies had a false negative rate of 1.8 percent.
Those more likely to be diagnosed with colon cancer despite a previous negative colonoscopy:
- Had a history of diverticular disease
- Had their colonoscopy exam done in an office setting rather than an ambulatory endoscopy center or hospital.
- Never had polyps removed.
Age, race, sex, or the number of colonoscopies performed by the doctor did not affect the risk for a missed cancer. However, doctors who diagnosed the most cancers had the lowest false negative rates.
Amanpal Singh and colleagues at the University of Texas Medical Branch in Galveston concluded,
Colonoscopy has around 95% sensitivity for colon cancer. Similar to the data from other countries, diverticular disease, office procedures and no prior polypectomy are risk factors for missed cancer in United States. However, there is no significant difference in diagnosis of right sided vs. left sided cancers.
SOURCE: Singh et al. Colon Cancer Miss Rates in Medicare Population, Abstract # 943, Digestive Disease Week 2009.
What This Means for Patients
Colonoscopy is not perfect. As reported in a Canadian study last year there were deaths from colon cancer even after recent negative colonoscopies.
People need to be aware of the symptoms of colon and rectal cancer and follow them up promptly with colonoscopy, even if they have had a previous negative exam.