There is no accurate blood or imaging diagnostic test for Parkinson's. Doctors make the diagnosis based on clinical examination - though this can be difficult as other conditions such as Alzheimer's disease can resemble PD and must first be excluded.
Misdiagnoses occur in up to 25 per cent of cases. Indeed, some claim that a definitive diagnosis is only possible on post mortem, when the fibrous tangles called Lewy bodies are found in that part of the brain that controls voluntary movement and produces the neurotransmitter dopamine.
The latest device used when the diagnosis is uncertain is the DaTSCAN. This brain-imaging technique is said to be able to differentiate parkinsonian syndromes from conditions such as essential tremor, which is much more common and requires different treatment (Pharm Vis, 2001; Summer: 30-2).
In a study to determine its diagnostic accuracy, DaTSCAN correctly identified 97 per cent of patients with PD (Mov Disord, 2000; 15: 503-10). However, other studies have shown a failure rate of 8 per cent (Arch Neurol, 2000; 57: 369-72).