A few short years ago these tests were said to be definitive, and touted as utterly accurate. I am one of the people with a negative scan (done in 2006). There have been others in the same position. Thanks Shetawk for posting these. You do not know how much it means to see this in print.

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The relative passages from Shetawks posts (my bold) from parkinson.org and PDF:

Recently, in studies that have attempted to diagnose Parkinson’s early in its course, researchers have found that a subset of patients thought to have Parkinson’s disease have turned up with negative PET or SPECT scans. These patients do not seem to develop the progressive symptoms of Parkinson’s disease. These findings are humbling, and they lend credence to the importance of following patients over long periods of time to ensure both accurate diagnosis, and also appropriate treatment.

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In the study of early Parkinson’s, the overall accuracy of DaTscan was equal to that of the accuracy of a physician’s diagnosis: both were 84 percent.
For people with more advanced PD, the overall accuracy of DaTscan was likewise identical to that of a physician’s diagnosis: both were 98 percent.