What does the term "test of choice" means in the U.S. healthcare system?

hi.

your question piqued my curiousity so i looked into it, and i think
that a "test of choice" refers to the most accurate method with which
to diagnose a patient.

for example,  this page here:
http://www.medicineonline.com/medicine/Diseases_and_Conditions/Cardiovascular_Disorders/Vascular_Disorders/Acute_arterial_occlusion_kidney/

identifies Kidney Ultrasound as "the initial diagnostic test of choice
to identify renal artery occlusions", as
"Physical examination is generally unrevealing unless the disorder has
persisted long enough to cause kidney failure."

i hope that helps, though you probably were looking for something more
specific, but just thought i'd try...you're my first comment, if it
wasn't glaringly obvious.

First, thank you for commenting. 

I'm looking for a formal definition from a government authority like
the Surgeon Genaral or the Department of Health and Human Services, or
any other formal authority like the World Health Organization.