98% accurate according to this site:
http://www.about-dementia.com/parkinsons/pd-diagnosis-treatment.php
The DaTSCAN: A new generation brain scan DaTSCAN (also called I123-FP-CIT), first launched in Europe in 2002, has been shown to diagnose PD with greater accuracy (up to 98 percent accuracy) compared with conventional scanning methods in patients exhibiting no PD symptoms other than tremors. According to a study outlined at the December 2002 Annual Meeting of the Radiological Society of North America (RSNA), DaTSCAN is particularly effective at distinguishing between Parkinson's disease and a less severe type of neurological disorder called essential tremor. In the words of David J. Tuite, MD: "Until the advent of DaTSCAN, the diagnosis of Parkinson's disease was primarily based upon clinical assessment...which has only an eighty percent accuracy rate." Although currently not widely available, DaTSCAN is an important advancement in the area of PD diagnosis.