According to a study recently published in the BMJ journal Gut, the presence of "invisible blood" in one's stool may indicate a higher risk of all-cause mortality.
The new research was led by first author Gillian Libby, of the Ninewells Hospital and Medical School in Dundee in the United Kingdom.
The corresponding author of the study is Robert Steele, a professor in the Division of Cancer at Ninewells Hospital and Medical School.
The presence of blood in the stool is a well-known test of colorectal cancer.
Perhaps less known is the fecal occult blood test (FOBT), which is a laboratory test used to detect occult, or hidden, traces of blood in a person's stool.
The test is commonly used to screen for colorectal cancer in seniors, or to detect adenomas, which are a precursor of colorectal cancer.
As Libby and colleagues explain in their paper, previous studies have found a correlation between the presence of blood in one's stool and the risk of premature mortality independently of bowel cancer.
But the magnitude of this link was not studied properly, and these previous studies did not account for factors such as gender, age, the use of certain drugs that may raise the risk of internal bleeding, or levels of socio-economic deprivation.
To correct this, Libby and team examined drug prescription, bowel cancer, and death registry data for almost 134,000 people followed clinically in 2000–2016.
Death risk raised by 58 percent
During the 16-year period, Libby and team found that 2,714 participants from the study sample tested positive for "unseen" blood in their stool.
The researchers followed their survival from the moment when they first took this test, which yielded positive results, to their death or the end of the study period.
The study found that old age, high levels of socio-economic deprivation, and being male all raised the chances of receiving a positive test result. Being prescribed drugs such as aspirin also raised the likelihood of testing positive for "unseen" blood.
Compared with those who tested negative, people who tested positive were almost eight times more likely to die from colorectal cancer.
However, the most surprising finding was that a positive test result also increased all-cause mortality risk by 58 percent.
More specifically, a positive FOBT was "significantly associated with increased risk of dying from circulatory disease, respiratory disease, digestive diseases (excluding [colorectal cancer]), neuropsychological disease, blood and endocrine disease."
This risk remained high even after adjusting for being male, older, and coming from a deprived social background.
The authors do caution that theirs is an observational study, with no immediate explanation for causality.
However, they suggest that generalized inflammation — which tends to manifest in the gut and through bleeding — may be the missing link.
"Although increased [unseen stool blood] cannot be a cause of death, it may reflect the reason why male gender, age, and deprivation are such strong risk factors," the authors write.
In a commentary accompanying the study, Prof. Uri Ladabaum — of the Stanford University School of Medicine in California — writes that the FOBT may offer unique insights into a person's general health.
"Perhaps more importantly," he states, "if occult blood in feces is a predictor of life expectancy and multiple [non-bowel cancer] causes of death, the inevitable next questions concern the implications for organized [bowel cancer] screening programs or opportunistic [bowel cancer] screening."