A proper understanding of the broad spectrum of clinical manifestations of PD is essential to the proper diagnosis of the disease. Because there is no definitive test for the diagnosis of PD, the disease must be diagnosed based on clinical criteria and clinical features. Rest tremor, bradykinesia, rigidity and loss of postural reflexes are generally considered the cardinal signs of PD.
　　Slowness of initiation and execution of voluntary movements, such as limb movements, facial expression or gait, characterizes bradykinesia. Diadochokinesis (the ability to perform rapid alternating movements) is usually slowed. The characteristic tremor is a low-frequency (4-6 Hz) resting tremor, but other tremor forms, such as an action tremor or a postural tremor may occur as the disease progresses. Rigidity becomes apparent at the clinical examination, when the passive movement of a limb is impaired by a wax-like resistance. In combination with the tremor frequency this results in the cogwheel phenomenon. Postural instability regularly appears in the course of the disease, most often in more advanced stages. It can be assessed with a pulling test.
　　Aside from these cardinal signs, clinical features include secondary motor symptoms (including hypomimia, dysarthria, dysphagia, sialorrhoea, micrographia, shuffling gait, festination, freezing, dystonia, glabellar reflexes), and nonmotor symptoms (including autonomic dysfunction, cognitive/neurobehavioral abnormalities, sleep disorders and sensory abnormalities such as anosmia, paresthesias and pain). Some of these nonmotor symptoms can occur very early or even precede the onset of motor features.
　　Other than using diagnostic criteria and clinical features, the diagnosis of PD can be assisted by several additional examinations. First of all is neuroimages. Although head MRI and CT are not helpful in establishing the diagnosis of PD, they have their places in excluding other parkinsonian syndromes. Imaging with radiolabeled ligands has markedly improved the functional diagnostics of PD. PET and SPECT techniques permit to visualize the pre- and postsynaptic compartment of the nigrostriatal projections and thus draw a semiquantitative picture about functionality of these pathways. In clinical use, they are mostly used for the differentiation of PD from atypical parkinsonian syndromes or from essential tremor.
　　Transcranial ultrasound has become a useful examination for the diagnosis in suspected PD. Hyperechogenicity in the substantia nigra occur in more than two thirds of all PD patients. Increased iron deposition and/or microglial activation may be responsible for this phenomenon.
　　Olfactory testing is an useful supplementary test in patients with suspected PD. Most patients (90 %) with PD show olfactory dysfunction early in the disease course. It helps confirming the diagnosis of PD, as well as differentiating PD from other Parkinsonian syndromes.
　　Genetic testing is helpful for the diagnosis and genetic counselling for familial PD, especially when a family history of PD exists or if disease manifestation occurs at a young age.
　　Responsiveness to dopaminomimetic therapy is an important supportive factor for the confirmation of the diagnosis of idiopathic PD and a lacking improvement of motor symptoms suggests the presence of an atypical parkinsonian syndrome, such as PSP, MSA or CBD. Clinical testing for dopaminergic response can be performed with levodopa (e.g. 200/50 mg levodopa/dopa decarboxylase inhibitor) by oral application or with apomorphine. The test is usually considered positive, if the patient improves by at least 30 % in the UPDRS.