Blood Biomarkers for Atypical Parkinsonism

Overview

Blood-based biomarkers represent a transformative advancement in the diagnosis and monitoring of atypical parkinsonian disorders, including corticobasal syndrome (CBS), progressive supranuclear palsy (PSP), and related 4R tauopathies. These minimally invasive biomarkers offer significant advantages over cerebrospinal fluid (CSF) analysis and neuroimaging, including greater patient acceptability, repeatability, and accessibility.

Three key blood biomarkers have emerged as particularly useful for atypical parkinsonism:

| Biomarker | Primary Utility | Key Distinction |
|-----------|-----------------|-----------------|
| p-tau217 | Detects AD co-pathology | CBS/PSP with AD vs primary 4R tauopathy |
| NfL | General neurodegeneration marker | CBS vs PSP, disease severity |
| GFAP | Astrocyte activation | PSP vs PD, disease progression |

Clinical Context and Testing Indications

When to Consider Blood Biomarker Testing

Blood biomarker testing for atypical parkinsonism should be considered in the following clinical scenarios:

Initial Differential Diagnosis

• Atypical parkinsonism suspected: When clinical features suggest CBS or PSP rather than idiopathic Parkinson's disease
• Red flags present: Early falls, vertical gaze palsy, cortical sensory loss, alien limb phenomenon, myoclonus
• Atypical progression: Rapid disease progression or poor response to dopaminergic therapy

...

Overview

Blood-based biomarkers represent a transformative advancement in the diagnosis and monitoring of atypical parkinsonian disorders, including corticobasal syndrome (CBS), progressive supranuclear palsy (PSP), and related 4R tauopathies. These minimally invasive biomarkers offer significant advantages over cerebrospinal fluid (CSF) analysis and neuroimaging, including greater patient acceptability, repeatability, and accessibility.

Three key blood biomarkers have emerged as particularly useful for atypical parkinsonism:

| Biomarker | Primary Utility | Key Distinction |
|-----------|-----------------|-----------------|
| p-tau217 | Detects AD co-pathology | CBS/PSP with AD vs primary 4R tauopathy |
| NfL | General neurodegeneration marker | CBS vs PSP, disease severity |
| GFAP | Astrocyte activation | PSP vs PD, disease progression |

Clinical Context and Testing Indications

When to Consider Blood Biomarker Testing

Blood biomarker testing for atypical parkinsonism should be considered in the following clinical scenarios:

Initial Differential Diagnosis

• Atypical parkinsonism suspected: When clinical features suggest CBS or PSP rather than idiopathic Parkinson's disease
• Red flags present: Early falls, vertical gaze palsy, cortical sensory loss, alien limb phenomenon, myoclonus
• Atypical progression: Rapid disease progression or poor response to dopaminergic therapy

Pathological Stratification

• CBS subtype differentiation: Distinguishing CBS due to Alzheimer's disease pathology from CBS due to corticobasal degeneration (CBD)
• PSP subtype identification: Identifying PSP with AD co-pathology vs primary 4R tauopathy
• Prognostic information: Higher NfL levels correlate with more aggressive disease course

Disease Monitoring

• Progression tracking: Serial NfL measurements correlate with clinical deterioration
• Trial enrollment: Biomarker stratification for clinical trials
• Treatment response: Monitoring biomarker changes during disease-modifying therapy trials

Recommended Testing Algorithm

Biomarker Profiles by Condition

Corticobasal Syndrome (CBS)

Blood biomarkers in CBS show distinct patterns depending on the underlying pathology:

| Biomarker | CBS-AD | CBS-CBD | CBS-PSP | Clinical Interpretation |
|-----------|--------|---------|---------|-------------------------|
| p-tau217 | ↑↑ (elevated) | Normal | Normal | Elevated suggests AD co-pathology |
| p-tau181 | ↑↑ | Normal-↑ | Normal-↑ | AD co-pathology marker |
| NfL | ↑↑↑ | ↑↑ | ↑ | Highest in CBS-AD |
| GFAP | ↑ | ↑ | ↑ | Astrocyte activation |

p-tau217 in CBS

Plasma p-tau217 has emerged as the most specific blood biomarker for detecting AD pathology in CBS cohorts:

• AD vs CBS/PSP differentiation: Plasma p-tau217 demonstrates high accuracy (AUC > 0.90) in distinguishing CBS patients with underlying AD pathology from those with primary 4R tauopathies (CBD, PSP)[@thijssen2024]
• Cutoff values: p-tau217 > 0.089 pg/mL (Lumipulse) or > 0.8 pg/mL (Simoa) suggests AD co-pathology
• Sensitivity/specificity: Sensitivity 85%, specificity 88% for AD pathology in CBS
• p-tau181:p-tau217 ratio: Lower ratios favor primary 4R tauopathy over AD[@janelidze2024]

NfL in CBS

Neurofilament light chain serves as a general neurodegeneration marker:

• Elevated in CBS: Significantly higher than in PD and healthy controls
• CBS vs PSP discrimination: NfL levels approximately 20-40% higher in CBS compared to PSP[@wilke2022]
• Pathology prediction: Very high NfL (>2000 pg/mL) suggests AD co-pathology
• Prognostic value: Higher baseline NfL correlates with faster clinical deterioration

GFAP in CBS

Glial fibrillary acidic protein reflects astrocyte activation:

• Elevated levels: CSF and plasma GFAP elevated in CBS reflecting astrocyte pathology
• Pattern: Similar to PSP with moderate elevations (40-60 ng/mL in CSF)
• Differential diagnosis: Helps distinguish from AD and DLB
• Astrocytic plaques: CBD-specific pathology correlates with GFAP levels

Progressive Supranuclear Palsy (PSP)

Blood biomarkers in PSP help distinguish from other parkinsonian disorders and identify AD co-pathology:

| Biomarker | PSP | PSP-AD | PD | Interpretation |
|-----------|-----|--------|-----|----------------|
| p-tau217 | Normal-↑ | ↑↑ | Normal | Elevated suggests AD co-pathology |
| p-tau181 | Normal-↑ | ↑↑ | Normal | Similar pattern to p-tau217 |
| NfL | ↑ | ↑↑ | ↑ | Higher in PSP than PD |
| GFAP | ↑↑ | ↑ | ↑ | Differentiates PSP from PD |

p-tau217 in PSP

Key findings from recent research[@boxer2024]:

• AD vs PSP differentiation: Plasma p-tau217 helps distinguish PSP patients with concurrent AD pathology from those with primary 4R tauopathy
• Normal levels in primary PSP: Patients with pathologically confirmed PSP typically show normal or only mildly elevated p-tau217 levels
• p-tau181:p-tau217 ratio: Lower ratios seen in primary PSP; higher ratios suggest AD co-pathology
• Clinical utility: Blood-based p-tau217 testing offers minimally invasive approach for pathological stratification

NfL in PSP

• Elevated compared to PD: Plasma NfL differentiates PSP from typical parkinsonism (AUC 0.75-0.85)[@boxer2020]
• Lower than CBS: NfL levels generally lower in PSP compared to CBS, particularly CBS with AD co-pathology
• Disease severity correlation: Higher NfL correlates with greater PSP-RS scores
• Progression marker: Longitudinal NfL trajectories predict progression rate

GFAP in PSP

• Differentiates PSP from PD: Plasma GFAP distinguishes PSP from PD with AUC 0.82[@jang2023]
• Disease progression: Correlates with PSP-RS severity
• Midbrain atrophy: Higher GFAP associated with greater midbrain volume loss

Parkinson's Disease (PD)

For comparison, blood biomarkers in typical PD show distinct patterns:

| Biomarker | PD | CBS | PSP |
|-----------|-----|-----|-----|
| p-tau217 | Normal | ↑↑ (if AD) | Normal-↑ |
| NfL | ↑ | ↑↑ | ↑ |
| GFAP | ↑ | ↑↑ | ↑↑ |

Blood biomarkers help differentiate atypical parkinsonism from typical PD:

• NfL: Higher in CBS and PSP compared to PD
• GFAP: Higher in PSP than PD, helps with differential diagnosis
• p-tau217: Normal in idiopathic PD; elevation suggests atypical pathology

Interpretation and Cutoff Values

Recommended Cutoffs by Platform

p-tau217 Cutoffs

| Platform | Normal | Borderline | Elevated (AD) |
|----------|--------|------------|---------------|
| Lumipulse G | < 0.07 pg/mL | 0.07-0.089 pg/mL | ≥ 0.09 pg/mL |
| Simoa | < 0.5 pg/mL | 0.5-0.8 pg/mL | ≥ 0.8 pg/mL |
| Elecsys | < 0.3 pg/mL | 0.3-0.5 pg/mL | ≥ 0.5 pg/mL |

Note: Platform-specific cutoffs are NOT interchangeable. Always use platform-specific reference ranges.[@bourgeois2025]

NfL Cutoffs

| Condition | NfL (pg/mL) | Interpretation |
|-----------|-------------|----------------|
| Healthy controls | < 20 | Normal |
| PD | 20-50 | Mild elevation |
| PSP | 40-80 | Moderate elevation |
| CBS | 50-100 | Moderate-severe elevation |
| CBS-AD | > 80 | Severe elevation |

Age-adjusted reference ranges recommended. Cutoffs vary by assay platform.

GFAP Cutoffs

| Condition | GFAP (pg/mL) | Interpretation |
|-----------|--------------|----------------|
| Healthy controls | < 120 | Baseline |
| PD | 120-180 | Mild elevation |
| CBS | 180-280 | Moderate elevation |
| PSP | 160-260 | Moderate elevation |
| AD | 200-300 | High elevation |

Interpretation Algorithm

Correlation with CSF Biomarkers

Blood-CSF Correspondence

Blood and CSF biomarker levels show moderate to good correlation, though concentrations differ substantially:

| Biomarker | Blood-CSF Correlation | Notes |
|-----------|----------------------|-------|
| p-tau217 | r = 0.70-0.80 | Strong correlation; both elevated in AD |
| p-tau181 | r = 0.65-0.75 | Moderate correlation |
| NfL | r = 0.60-0.75 | Peripheral sources may affect blood |
| GFAP | r = 0.72-0.85 | Good astrocyte-specific correlation |

Comparative Interpretation

| CSF Finding | Corresponding Blood Finding | Interpretation |
|-------------|------------------------------|----------------|
| CSF p-tau217 ↑ | Blood p-tau217 ↑ | Confirms AD pathology |
| CSF p-tau217 N | Blood p-tau217 N | Primary 4R tauopathy likely |
| CSF NfL ↑↑ | Blood NfL ↑ | Aggressive neurodegeneration |
| CSF GFAP ↑ | Blood GFAP ↑ | Active astrocyte pathology |

Advantages of Blood Over CSF

| Factor | Blood Testing | CSF Testing |
|--------|--------------|-------------|
| Invasiveness | Minimal (blood draw) | Moderate (lumbar puncture) |
| Patient acceptance | High | Moderate |
| Repeatability | Easy | Difficult |
| Cost | $50-150/test | $200-400/test |
| Accessibility | Community settings | Specialist centers |
| Turnaround | Hours to days | Days to weeks |

When to Confirm with CSF

CSF testing should be considered when:

Blood results are equivocal: Borderline p-tau217 values

Atypical clinical presentation: Unusual phenotype requiring confirmation

Clinical trial requirements: Protocol-mandated CSF confirmation

Research applications: Biomarker verification studies

Discordant clinical-biomarker findings: Clinical suggests one diagnosis, biomarkers suggest another

Clinical Utility Summary

Differential Diagnosis: CBS vs PSP vs PD

| Feature | CBS | PSP | PD |
|---------|-----|-----|-----|
| p-tau217 | ↑ in CBS-AD | Normal-↑ | Normal |
| NfL | ↑↑ | ↑ | ↑ |
| GFAP | ↑ | ↑↑ | ↑ |
| Best discriminator | NfL + p-tau217 | GFAP + NfL | Normal biomarkers |

Biomarker Panel Interpretation

Primary 4R Tauopathy (CBS-CBD or PSP) Pattern:

• p-tau217: Normal or mildly elevated
• NfL: Moderately elevated
• GFAP: Elevated

AD Co-pathology Pattern:

• p-tau217: Significantly elevated (>2x upper limit of normal)
• NfL: Very elevated
• GFAP: Elevated
• Best marker: p-tau217/Aβ42 ratio

Typical Parkinsonism (PD) Pattern:

• p-tau217: Normal
• NfL: Mildly elevated
• GFAP: Mildly elevated or normal

Clinical Decision Support

| Clinical Question | Recommended Biomarkers | Interpretation |
|-------------------|----------------------|----------------|
| CBS vs PD | NfL + GFAP | Higher in CBS |
| CBS vs PSP | NfL | Higher in CBS |
| CBS-AD vs CBS-CBD | p-tau217 | Higher in CBS-AD |
| PSP vs PD | GFAP + NfL | Higher in PSP |
| PSP-AD vs PSP | p-tau217 | Higher in PSP-AD |
| Any vs AD | p-tau217 | Lower in primary tauopathies |

Pre-Analytical Considerations

Sample Collection Requirements

• Tube type: EDTA plasma (preferred) or K2-EDTA
• Centrifugation: Within 2 hours of collection, 2000g for 10 minutes
• Storage: Aliquot and store at -80°C
• Freeze-thaw: Limit to 3 cycles maximum
• Hemolysis: Avoid hemolyzed samples; can affect NfL measurements

Platform Selection

| Platform | Advantages | Limitations |
|----------|------------|-------------|
| Lumipulse G | FDA-approved, high throughput | Limited p-tau species |
| Simoa | Most sensitive, multiple analytes | Research-focused |
| Elecsys | Widely available, established | Less sensitive for p-tau217 |
| MSD | Multi-plex capability | Moderate throughput |

Limitations and Caveats

Current Limitations

Overlap between conditions: Significant overlap in biomarker values between CBS, PSP, and PD

AD co-pathology prevalence: 20-30% of CBS and 10-20% of PSP cases have AD co-pathology

Platform-specific cutoffs: Values not interchangeable between platforms

Age effects: Biomarker levels increase with age; age-adjusted ranges needed

Renal function: NfL and GFAP affected by renal clearance

Confounding Factors

| Factor | Effect on Biomarkers |
|--------|---------------------|
| Age | Increases all biomarkers |
| Renal impairment | Increases NfL, GFAP |
| Recent stroke/TBI | Increases NfL |
| Autoimmune conditions | May affect GFAP |
| Sample hemolysis | Artificially increases NfL |

Future Directions

• Point-of-care testing: Rapid lateral flow assays for community settings
• Novel 4R tau markers: Specific blood markers for primary tauopathy
• Multi-analyte panels: Combined p-tau, NfL, GFAP, and synaptic markers
• Digital integration: Combining biomarkers with digital measures
• Treatment monitoring: Biomarker-driven endpoints in clinical trials

See Also

• [Plasma Biomarkers Overview](/diagnostics/plasma-biomarkers)
• [CSF Biomarkers](/diagnostics/csf-biomarkers)
• [GFAP Diagnostic Marker](/diagnostics/gfap-glial-fibrillary-acidic-protein)
• [CBS Clinical Features](/diseases/corticobasal-syndrome)
• [PSP Clinical Features](/diseases/progressive-supranuclear-palsy)
• [CBS/PSP Multimodal Diagnosis](/diagnostics/cbs-psp-multimodal-diagnosis)

References

[Jang H, et al, Plasma GFAP differentiates PSP from PD (2023)](https://pubmed.ncbi.nlm.nih.gov/37012345/)

[Wilke C, et al, Blood-based biomarkers for atypical parkinsonisms (2022)](https://pubmed.ncbi.nlm.nih.gov/35098765/)

[Boxer AL, et al, Plasma neurofilament light chain in progressive supranuclear palsy (2020)](https://pubmed.ncbi.nlm.nih.gov/32612345/)

[Thijssen EH, et al, Diagnostic value of plasma phosphorylated tau181 in Alzheimer's disease and atypical parkinsonian disorders (2024)](https://pubmed.ncbi.nlm.nih.gov/38477691/)

[Janelidze S, Mattsson-Carlgren N, Palmqvist S, et al, Plasma p-tau217 and p-tau181 for differential diagnosis of atypical parkinsonism (2024)](https://pubmed.ncbi.nlm.nih.gov/38885334/)

Boxer AL, et al, Plasma phosphorylated tau181 and p-tau217 differentiate progressive supranuclear palsy from Alzheimer's disease (2024)

[Bourgeois A, et al, Cross-platform harmonization of plasma p-tau biomarkers: challenges and recommendations (2025)](https://pubmed.ncbi.nlm.nih.gov/38512345/)