Here is a common definition of what the mRS is meant to do and a description of the 7 categories of the mRS copied from MEDCALC.
“Measures the degree of disability or dependence in the daily activities of people who have suffered a stroke or other causes of neurological disability.”
mRS Category Description
|0| No symptoms at all
|1| No significant disability despite symptoms; able to carry out all usual duties and activities
|2| Slight disability; unable to carry out all previous activities, but able to look after own affairs without assistance
|3| Moderate disability; requiring some help, but able to walk without assistance
|4| Moderately severe disability; unable to walk and attend to bodily needs without assistance
|5| Severe disability; bedridden, incontinent and requiring constant nursing care and attention
|6| Dead
The mRS was developed as a global measure of disability in patients who have had a stroke. The specific wording for categorization at, for example, a time 3 months after an acute stroke refers, implicitly (category 1) and explicitly (category 2), to a time before the acute stroke.
You posit that:
It is very, very unlikely, not to say semi-impossible, for a patient to improve their mRS to the point of getting better than pre-stroke mRS after a hospitalization due to stroke.”
This statement is surely true considering the mRS category of 6 (Dead) pre-stroke. Indeed, the mRS category of 6 obviously cannot exist pre-stroke.
It also seems, as you point out, highly unlikely even impossible, for a person admitted to the hospital with an acute stroke (or suffering an acute stroke while in the hospital for something else) and who was, pre-stroke, “severely disabled; bedridden; incontinent and requiring constant nursing care and attention”–(mRS category 5)–to move to a lower mRS category after an acute stroke.
However, as I understand it, patients eligible for your analysis have been selected to have a mRS<=2 at the time of their acute stroke.
You describe these patients as:
“patient lives independently”
But this is not the definition of mRS categories <=2.
In my opinion, a clinician or someone else who attempts to “assign” a mRS category of 0, 1, or 2 using the standard descriptions in the table to a patient with an acute stroke is going to be more than a bit confused.
What does it mean to have “no symptoms at all”–the descriptor for mRS category 0—at the time of an acute stroke? Does it mean no symptoms of neurologic disease? More importantly, a person with no symptoms of neurologic disease probably has not had an acute stroke.
Category 2 doesn’t make a lot of sense when considering status at the time of admission to the hospital with an acute stroke because it says “….unable to carry out all previous activities.” The patient’s functional status will evolve over time after an acute stroke. It would be impossible for a rater assessing the patient at the time of an acute stroke to know at this “baseline” time whether, at later time, the patient will be able to carry “all previous activities.”
If mRS categories 0, 1, and 2 can be redefined as “patient lives independently” at the time of the acute stroke based on empiric data, in my opinion, it logical to simply combine them for the purposes of your modeling.
Or simply point out that applying the standard descriptors of categories of the mRS to patients with an acute stroke at the time of the acute stroke does not make sense and may have confused the raters, leading to misclassification that invalidates the use of the individual categories 0, 1, and 2 as “baseline” measures of functional status.
I agree, the categories 0, 1, 2 as separate categories at some “baseline” are nonsense.