The degree of cardiac sympathetic denervation in PD patients is positively associated with disease progression. The relative ratio of radioisotope uptake into the heart to that in the apparent discrepancy adora2b kinetics function mediastinum in MIBG scintigraphy is effective for making a differential diagnosis between PD and MSA in the early stage of the disease. Comparing the diagnostic accuracy of the cardiac MIBG test and that of the putaminal ADC test, it is reported that the latter is superior for differential diagnosis between MSA-P and PD ; however, this has not been investigated in the early stage of the disease. The purpose of the present study was to investigate the diagnostic accuracy of these tests when applied to early-stage patients. The most discriminative cut-off points for ADC and MIBG scintigraphy in MSA-P and PD patients were determined, and the diagnostic accuracy of the tests was assessed by applying these cut-off points to patients with disease duration #3 years. A retrospective study was conducted to investigate diagnostic accuracy of the putaminal ADC test for MSA-P and cardiac MIBG test for PD. Referral standard diagnosis of PD and MSA-P was made according to the United Kingdom Parkinson’s Disease Society Brain Bank Clinical Diagnostic Criteria and the second consensus clinical diagnostic criteria, respectively. According to these criteria, the referral diagnosis was made carefully by three expert neurologists who were masked to the result of ADC and MIBG tests. Collecting all medical records until December 2012 for the referral diagnostic tests, the diagnosis was made when the diagnoses by three experts were consistent, and the diagnosis was undetermined when they were inconsistent. Based on the referral diagnostic criteria, the specificity, sensitivity, positive likelihood ratio, negative likelihood ratio, and the area under the receiver-operator characteristic curve of the two index tests, the putaminal ADC test and cardiac MIBG test, were obtained and the most discriminating cut-off points were determined using the ROC curves. When applying these cut-off points to test results obtained from early patients with disease duration of #3 years, the diagnostic accuracy was calculated to investigate early diagnostic usefulness. We enrolled 260 consecutive patients with muscular rigidity, hand or leg tremor, or slowed movements who had undergone both ADC and MIBG scintigraphy at the Department of Neurology of Utano National Hospital between January 2001 and October 2010. Patients with a history of diabetes mellitus, myocardial infarction, current heart diseases including heart failure and cardiomyopathy, and current use of monoamine oxidase B inhibitors, droxidopa or tricyclic antidepressants were excluded, which can interfere with MIBG imaging, and patients with MRI findings of other putaminal lesions were also excluded, which can influence the ADC test. The study was approved by the Bioethics Committee of Utano National Hospital, and written informed consent was obtained from each participant. Patients’ characteristics including age at study enrollment, sex, age of onset, disease duration, disease severity, presence of orthostatic hypotension, and levodopa daily dose were collected. The definition of orthostatic hypotension was according to UPDRS IV. To determine the normal range of the putaminal ADC in our hospital, age-matched controls were recruited. Concerning the cardiac MIBG test, we referred to our previous study due to the risk of radiation exposure and estimated the mean H/M ratio to be 2.45 in control patients. The putaminal ADC test is based on the diffuse rarefaction of the putamen, and consistent with previous studies, the putaminal diffusivity was significantly elevated in patients with MSA-P. The cardiac MIBG test is based on peripheral sympathetic nerve terminals, and cardiac accumulation of MIBG was significantly reduced in patients with PD.