Thread by @awgaffney

Many diseases are defined along a spectrum, by continuous variable(s).

While the presence (or absence) of such diseases may be obvious at either extreme of that spectrum, there is no "true" objective threshold at which no disease becomes disease.

jamanetwork.com/journals/jamai…

Instead, the line is drawn by human beings, and the decision of *where* to draw the line often hinges on pragmatic (utilitarian) considerations, namely:

At what threshold do the benefits from diagnosis/treatment outweigh its harms? Cochrane said something like this long ago.

That is how, to some extent anyway, we seem to have decided where to draw the line between high blood pressure and normal BP:

At what blood pressure do the gains of blood pressure lowering exceed its harms? (Anyway that's how it should be defined, in my opinion).

In that sense, "What is high blood pressure" is not a thorny metaphysical or even physiological question.

It is a pragmatic classification, effectively equating to antihypertensive-medication-deficiency (or, more diplomatically, "who could benefit from blood pressure lowering?).

Obviously diseases vary enormously, and hypertension (which some characterize as a risk factor and not a disease although I find the distinction unpersuasive) is a unique case. Yet it speaks to the necessary pragmatism approach of drawing of diagnostic thresholds — as does ...

... chronic kidney disease, as explored in the commentary linked to above, even if the diagnostic threshold = therapeutic threshold equation does not play out so neatly (and indeed, it rarely does so as neatly as with hypertension, & even there it is not so neat).

And obviously, there are a multitude of potential benefits to diagnosis even when there is no therapy. When diagnostic thresholds must be drawn along a continuous spectrum, they must be drawn in consideration of the full spectrum of potential benefits and harms.

These thoughts are still a work in progress — and I'm curious to hear other opinions.

Worth noting a seemingly bizarre implication here. We tend to think that diseases are “discovered,” diagnostic thresholds defined, & then therapies developed. Yet if above is true, the development of a new therapy could - should - change the boundaries of disease.

In other words, if new types of blood pressure medications were invented that, hypothetically, reduced risk of stroke and mortality at a *lower* BP threshold than existing medications, arguably the diagnostic threshold should be lowered, or at least that would be a discussion.

The diagnosis > therapy relationship would, to some extent, be reversed. Therapy itself would (in a non-problematic way) come to shape diagnostic boundaries.