The Design Flaws in the Bristol Stool Scale
I’ve been reading medical papers reporting study results based on the Bristol Stool Scale, pictured above. The researchers often comment that the study patients confused category one and category five. I’m not surprised. The researchers typically blame it on the patients but I think the problem is with the scale.
This scale is trying to measure two separate facts that are not always correlated. The first fact that it tries to measure is the physical description of the stool. A purpose of the chart is to make it easy for patients to categorize their own poop according to the physical descriptions in the scale. The second fact it is trying to measure is how difficult or easy the stool is to pass. This is where the problem with the scale lies.
There is not necessarily a correlation between the physical nature of the stool and how difficult or easy it is to pass. In some people, number one passes easily despite the scale allowing classification of number one only if it is difficult to pass. Sometimes, in persons who have chronic diarrhea, number seven can be difficult to pass. There have been times when I have experienced severe pain in the lower abdomen and lower back accompanied by severe nausea. This occurs towards the end of a large quantity of diarrhea, when the body is trying to expel every last remnant. It’s a bit like the dry heaving after vomiting but at the other end of the system.
I suspect the problems result from the scale being designed by researchers whose focus was on constipation, without consulting researchers whose focus was on chronic diarrhea. One clue of the design bias is the depiction of three categories with very fine distinctions for constipation but only one category for diarrhea. Another clue indicating design bias is the failure to account for the fact that a healthy bowel movement typically occurs at a time that is convenient but diarrhea tends to come on suddenly when not convenient.
To address these design flaws, I propose that the Bristol Stool Scale be replaced with two independent scales of 5 categories each:
Scale of Physical Stool Descriptions
- Solid pebble shapes
- Solid sausage shape
- Soft sausage shape
- Soft, mushy pieces
Scale of Stool Evacuation
A. Difficult to pass, with pain
B. Somewhat difficult to pass, without pain
C. Easy to pass, and when convenient
D. Need arises when not convenient, but allows a few minutes to get to a toilet
E. Need arises suddenly, and allows less than a minute to get to a toilet
A questionnaire should allow for the fact that a single bowel movement can start in one category and end in another on either scale. And the transition might not be directly to a neighbouring category. The severe diarrhea described above started as 5E but ended up at 5A. Sometimes constipation starts at 1A but ends at 4C.
Neither these scales nor the Bristol Stool Scale addresses quantity of stool. Quantity would provide additional useful information relevant to speed of passage of food through the entire digestive system. Patient description of quantity is subjective. One patient’s large amount may be another patient’s normal amount. Objective quantity is difficult to measure unless patients are asked to poop into a separate measuring container. But that might cross a line beyond which a patient is unwilling to participate. It’s one thing to answer a few questions – but imagine having to have a measuring container handy, and how to poop into it (put it on the floor and squat over it?), and the mess of disposing of the contents and cleaning it ready for the next time. And what measuring yardstick? – millilitres? Grams? On the bathroom scale? Don’t go there.