Subject: What exactly does Erythrocyte Sedimentation Rate test for and how?

Part of what I do in my profession is refer patients to other health 
professionals when a systemic disease is suspected through ocular 
examination, such as GCA (Giant Cell Arteritis).  I have learned 
in my clinical training that a Erythrocyte Sedimentation Rate or ESR 
test is used to diagnose some diseases like the aforementioned GCA.  
However, I cannot find in any laboratory testing manual or medical 
diagnoses texts any specific information on ESR. I was just curious as 
to what specifically is being tested and how?

Re: What exactly does Erythrocyte Sedimentation Rate test for and how?