Blood Tests for Parkinson's Disease, Alzheimer's Disease, and Dementia
For Alzheimer's disease, plasma biomarkers including p-tau217, p-tau181, and Aβ42/Aβ40 ratio are now clinically available and recommended for detecting amyloid pathology in patients with cognitive impairment, though they should be confirmed with CSF or PET testing when disease-modifying treatments are being considered 1, 2.
Current Blood Biomarker Landscape
For Alzheimer's Disease
The most validated blood biomarkers for AD include:
Plasma p-tau variants (p-tau217, p-tau181, p-tau231): These are the highest-performing blood tests for AD, with p-tau217 showing slightly superior performance in differentiating AD dementia from other dementias 1. In the dementia stage, high-performing plasma p-tau assays can differentiate AD from other dementias with similar accuracy to CSF and PET markers 1.
Plasma Aβ42/Aβ40 ratio: Most useful for detecting cerebral amyloid pathology, particularly when combined with APOE genotype in cognitively unimpaired and MCI individuals 1.
Plasma GFAP (glial fibrillary acidic protein): Reflects astrocytic activation and can predict cognitive decline and AD dementia development 1.
Plasma NfL (neurofilament light): A non-specific marker of neurodegeneration. When elevated with normal p-tau, it may indicate non-AD dementia like frontotemporal dementia 1.
For Parkinson's Disease
There are currently no validated blood biomarkers specifically for diagnosing Parkinson's disease 2. The diagnosis remains clinical. However, CSF alpha-synuclein seed amplification assay is emerging as a biomarker for Lewy body pathology 2.
For Other Dementias
Blood biomarkers are less developed for non-AD dementias. The pattern of elevated NfL with normal p-tau may suggest frontotemporal dementia or corticobasal degeneration 1.
Clinical Application Algorithm
Step 1: Basic Laboratory Panel (Tier 1 - Required for All)
Before any specialized biomarker testing, obtain 3:
- Complete blood count with differential
- Complete metabolic panel (Chem-20) with renal/hepatic panels, electrolytes, glucose, calcium, magnesium, phosphate
- Thyroid-stimulating hormone (TSH)
- Vitamin B12 level
- Homocysteine level
- C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR)
Step 2: When to Use Blood Biomarkers for AD
Current appropriate use 1:
- Only in symptomatic patients (MCI or dementia) at specialist memory clinics
- After comprehensive clinical assessment ruling out other causes
- Results should be confirmed with CSF or PET when available, especially if disease-modifying treatments are considered
Specific clinical scenarios 1:
- Distinguishing AD dementia from other dementias: Use plasma p-tau217 or p-tau181 as first-line blood test
- Detecting AD pathology in MCI: Combine plasma Aβ42/Aβ40 with p-tau181 or p-tau217
- Predicting AD dementia development: Plasma p-tau181, p-tau217, p-tau231, or GFAP in patients with MCI or subjective cognitive decline
Step 3: Performance Standards
Blood biomarkers should meet these criteria 4:
- Sensitivity ≥90%
- Specificity ≥90% for confirmatory testing (≥85% for triaging tests)
- Less than 5% discordance when blood is collected and analyzed on different occasions
Critical Limitations and Caveats
Not Ready for Primary Care
Blood biomarkers for AD have not been validated in primary care settings and should not be used there yet 1. The patient population in primary care is more heterogeneous with more comorbidities, and the prevalence of neurodegenerative diseases is lower, which significantly affects test performance 1.
Parkinson's Disease Diagnosis Remains Clinical
No blood test can diagnose Parkinson's disease 2. The diagnosis is based on clinical criteria including motor symptoms (bradykinesia plus rigidity or tremor), response to dopaminergic therapy, and absence of atypical features.
Confounding Factors
Be aware that blood biomarker results can be affected by 1:
- Kidney disease (affects NfL and potentially other markers)
- Traumatic brain injury (affects GFAP)
- Cerebrovascular disease (affects GFAP)
- Peripheral neuropathy (may affect GFAP)
Not Standalone Tests
Blood biomarkers should never be used as standalone diagnostic tests 4. They must be interpreted within the full clinical context after ruling out other causes of cognitive impairment including depression, medication effects, metabolic disorders, and structural brain lesions 2.
Optimal Biomarker Combinations
For predicting AD dementia in MCI patients: Combine plasma p-tau with NfL, and consider adding brief cognitive tests and APOE genotype for algorithms that perform as accurately as CSF-based approaches 1.
For detecting amyloid pathology in early disease: Plasma Aβ42/Aβ40 combined with APOE genotype and p-tau shows best performance 1.
The Bottom Line
Blood biomarkers represent a significant advance for Alzheimer's disease diagnosis in specialty settings, but they are not yet ready for widespread use in primary care or for diagnosing Parkinson's disease. The basic metabolic and hematologic laboratory panel remains essential first-line testing for all patients with cognitive symptoms to exclude reversible causes.