Until we have some research proven biomarkers or diagnostic tests for PD, it will never be valid assessment measures. Most of the main assessment test of every trial design uses the UPDRS. ( see this information - http://www.alz.washington.edu/NONMEM.../VER1_2/b3.pdf )
It has its problems but it has been used 20+ years.

Most all research testing is based on observed performance, leaving too much chance or rater bias (subjective opinion). Our concerns are real when it comes to accurate diagnosis.
Peggy