False negative fecal occult blood test may be associated with increased mortality from colorectal cancer

Fecal Occult Blood Test (FOBT) is an accepted screening test for colorectal cancer (CRC). It has been shown to decrease mortality by up to 30%. The outcome of screening failures has not been adequately studied.

The purpose of this study was to assess the outcome of patients who were diagnosed with CRC after a false negative FOBT.

We identified all consecutive CRCs from pathology reports between 2005 and 2010. Patients were divided according to their FOBT result. Those who became positive were compared to patients who remained negative.

Altogether 401 CRCs were identified. Of those, 202 never performed a FOBT. At least one negative FOBT was performed by 133 individuals (67%). Of these, 76 remained negative (false negatives, FN) and 57 became positive (positive conversion, PC, controls). The prevalence of metastatic disease was threefold higher among the FNs as compared to the PC group (16 [22.2%] vs. 4 [7.5%], P=0.022). All-cause mortality was also significantly higher among FNs versus PCs (24 [31.6%] vs. 5 [8.8%], P=0.001); in Cox regression analysis of survival (covariates: FNs vs. PC, gender, age, medications and co-morbidities) FNs had increased mortality compared to the PC (HR 2.929, P=0.033, CI 95% 1.092-7.858). No statistically significant difference was found regarding all primary end points when comparing the FN and the "No test" group.

These data disclose a particular risk of FOBT as a screening test. A subgroup of patients with "false" negative tests may have increased morbidity and mortality. Efforts should be made to recognize and characterize this high-risk group.