Thanks for your replies.

Diego24, your simple example illustrates the point well. You mention a condition of "low blood pressure" and go on to point out that "The biomarker is measuring that your blodd [sic] pressure is low". By defining the disease in terms of a body state appears to make it easy to identify the biomarker. But wait. You describe the symptom as "feeling dizzy if you get out of your chair". This could be postural hypotension: a failure of the body to automatically return blood pressure to normal. From this perspective we can see that blood pressure alone would not be a sufficient biomarker. Of course, the next step would be to ask what causes the postural hypotension. This can be a symptom of Parkinson's Disease.

indigogo, I too support the search for biomarkers, but I also see a role for objective symptom based tests. For instance, I find the work of Little very interesting. He has a computer system that analyses voice recordings to diagnose and measure the progression of PD.

John