'Invisible' stool blood linked to heightened risk of death from all causes

In particular, it is associated with a heightened risk of dying from circulatory, respiratory, digestive, blood, hormone and neuropsychological diseases, and other types of cancer, the findings indicate.

A test to pick up the presence of unseen blood in the stools, known as the faecal occult blood test, or FOBT for short, is currently used to screen for bowel cancer or its precursor, polyps, in older people.

Previous research has indicated that stool blood might predict life expectancy, independent of its association with bowel cancer. But potentially influential factors, including drugs that might predispose to internal bleeding -- aspirin, for example -- were not accounted for.

To try and address this, the researchers drew on linked prescribing, bowel cancer screening, and death registry data for nearly 134,000 people in Tayside, Scotland, from March 2000 to the end of March 2016.

In all, 131,207 people, aged between 50 and 74, tested negative for unseen stool blood and 2714 (just over 2%) tested positive during this period. Their survival was tracked from the date of this first test until death or the end of March 2016, whichever came first.

Older age, increasing levels of deprivation, and male gender were associated with a greater likelihood of a positive test result. Prescriptions for aspirin or other drugs that boost the risk of digestive tract bleeds were also more likely to be associated with a positive test result.

Overall, people who had a positive FOBT result were nearly eight times as likely to die of bowel cancer as those who tested negative, after taking account of gender, age, deprivation, and drug treatment.

But more surprising was that a positive FOBT result was also associated with a 58 percent heightened risk of death from all causes other than bowel cancer.

It was associated with a significantly greater risk of death from circulatory, respiratory, digestive tract, neuropsychological, blood and hormone diseases, as well as other types of cancer, after taking account of potentially influential factors.

The researchers point out that older age, male gender, and increasing levels of deprivation are themselves risk factors for death from all causes. Yet even after adjusting for these factors, a positive FOBT result was still strongly associated with early death.

"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," they suggest.

This is an observational study, and as such, can't establish cause. And there are no obvious explanations for their findings, point out the researchers.

But they speculate that generalised inflammation, manifest in gut inflammation and bleeding, may provide the missing link, particularly as there is good evidence that most solid cancers and Alzheimer's disease develop against a background of chronic/systemic inflammation.

They go on to suggest that a positive FOBT result might be used to alert bowel cancer screening programme participants who don't have the disease -- and around half of those who test positive don't -- to the possibility that they might be at risk of other potentially life-limiting illness, and the need for a healthier lifestyle and/or preventive drug treatment.

In a linked commentary, Professor Uri Ladabaum of Stanford University School of Medicine, writes that hidden stool blood "may be telling us more than we might have thought. If the eye is the window to the soul, is a faecal test the window to general health?"

He continues: "Perhaps more importantly, if occult blood in faeces is a predictor of life expectancy and multiple [non-bowel cancer] causes of death, the inevitable next questions concern the implications for organised [bowel cancer] screening programmes or opportunistic [bowel cancer] screening."