What Does a Positive Nuclear Medicine DAT Scan Mean?
A positive (abnormal) DAT scan indicates loss of dopaminergic neurons in the nigrostriatal pathway, confirming the presence of a neurodegenerative parkinsonian syndrome such as Parkinson's disease, multiple system atrophy, progressive supranuclear palsy, or dementia with Lewy bodies. 1, 2
Understanding DAT Scan Results
Normal vs. Abnormal Findings
- Normal DAT scan: Shows symmetric, comma-shaped uptake in both the caudate nuclei and putamina bilaterally 3
- Abnormal (positive) DAT scan: Demonstrates loss of the normal comma shape with reduced radiotracer uptake in the striatum, indicating reduced dopamine transporter density from neuronal degeneration 3
- The abnormality typically begins in the putamen and progresses to involve the caudate nuclei 1, 2
What a Positive Scan Confirms
A positive DAT scan confirms dopaminergic neuronal loss but does NOT specify which parkinsonian syndrome is present. 2, 3 The scan cannot differentiate between:
- Idiopathic Parkinson's disease
- Multiple system atrophy
- Progressive supranuclear palsy
- Corticobasal degeneration
- Dementia with Lewy bodies
All of these conditions show abnormal DAT scans because they all involve degeneration of dopaminergic neurons. 4
What a Positive Scan Rules Out
An abnormal DAT scan effectively excludes non-neurodegenerative causes of parkinsonism 4, 5:
- Essential tremor (79% of suspected cases had normal scans) 5
- Drug-induced parkinsonism 1, 4
- Vascular parkinsonism (unless focal basal ganglia infarction is present) 4
- Psychogenic parkinsonism 4
Clinical Context and Diagnostic Accuracy
When DAT Scans Are Most Reliable
- Highest correlation: When the prescan diagnosis is essential tremor—79% of suspected ET cases showed normal scans 5
- Moderate correlation: When neurodegenerative parkinsonism is suspected—57% of these cases showed abnormal scans 5
- Lowest correlation: When psychogenic disorder is suspected—only 47% showed normal scans, indicating diagnostic uncertainty in this population 5
Sensitivity and Specificity
- For differentiating dementia with Lewy bodies from Alzheimer's disease: 78% sensitivity and 90% specificity 3
- Dopamine loss is detectable even in the earliest clinical presentations of true parkinsonism 4
Critical Clinical Implications
Treatment Decisions
A positive DAT scan strongly influences treatment initiation: Only 4% of patients with abnormal scans remained off anti-parkinsonian medications, whereas 24% of patients with normal scans were still on anti-parkinsonian medications despite the negative result. 5 This demonstrates that clinicians appropriately use positive DAT scans to justify starting dopaminergic therapy.
Limitations You Must Understand
DAT scans should be regarded as supportive rather than diagnostic of dopamine-deficient parkinsonism. 3 The scan:
- Does not monitor disease progression in established Parkinson's disease 2
- Cannot assess effectiveness of neuroprotective agents reliably, as DAT expression can be modified by anti-parkinsonian medications 6, 7
- Should never be ordered without first obtaining MRI brain to exclude structural causes like vascular parkinsonism, normal pressure hydrocephalus, or space-occupying lesions 2
Common Pitfalls to Avoid
- Never interpret a positive DAT scan as confirming "Parkinson's disease" specifically—it only confirms a neurodegenerative parkinsonian syndrome 2, 3
- Do not use DAT scans to monitor progression in patients with established diagnosis 2
- Always obtain MRI brain first before ordering DAT scan 2
- Recognize that DAT expression can be therapeutically modified by anti-parkinsonian drugs, potentially confounding interpretation 6, 7
- Be cautious when prescan diagnosis is psychogenic disorder—scan results correlate poorly in this population (only 47% showed expected normal results) 5