What blood tests are recommended to evaluate for Alzheimer’s disease or risk of dementia?

Blood Tests for Alzheimer's Disease and Dementia Risk

Blood-based biomarkers, particularly phosphorylated tau proteins (p-tau217 and p-tau181), should be used as confirmatory tests in specialized memory care settings for patients with objective cognitive impairment, achieving diagnostic accuracy exceeding 90% sensitivity and specificity when compared to CSF analysis or amyloid PET. 1

Key Blood Biomarkers with Established Diagnostic Value

The most recent 2025 Alzheimer's Association guidelines establish a clear hierarchy of blood-based biomarkers (BBMs) for clinical use 1:

Core AD Biomarkers (Highest Priority)

• Phosphorylated tau-217 (p-tau217): Demonstrates exceptional accuracy with AUC >0.93 and 91% positive predictive value, with plasma levels increasing >8.5% annually during preclinical stages 2, 3
• Phosphorylated tau-181 (p-tau181): Consistently high diagnostic performance (AUC up to 0.93) 3
• Amyloid-beta 42/40 ratio (Aβ42/40): Moderate to high accuracy for preclinical detection (AUC 0.69-0.94) 3

Supporting Biomarkers

• Glial fibrillary acidic protein (GFAP): Strong performance as marker of astrocytic activation (AUC up to 0.87) 4, 3
• Neurofilament light chain (NfL): Moderate diagnostic value (AUC up to 0.79), better for predicting progression than early diagnosis 4, 3

When to Order Blood Biomarkers

Appropriate Clinical Scenarios

Blood biomarkers should be ordered for:

• Patients with objective cognitive impairment (MCI or dementia) presenting to specialized memory care settings 1
• Situations where determining AD pathology presence would change clinical management
• Triaging patients before more invasive CSF analysis or expensive amyloid PET imaging 1

Critical Contraindications

Do NOT order blood biomarkers for:

• Asymptomatic individuals without cognitive complaints 5
• Population screening programs 5
• Direct-to-consumer testing outside clinical context 5
• Before comprehensive clinical evaluation by a healthcare professional 1

Diagnostic Accuracy Standards

The 2025 guidelines establish minimum acceptable thresholds 1:

• Confirmatory testing: ≥90% sensitivity AND ≥90% specificity
• Reference standards: CSF AD biomarkers, amyloid PET, or AD neuropathology

When blood biomarkers meet these thresholds, they can substitute for CSF analysis or amyloid PET as confirmatory tests 1.

Optimal Testing Strategy

Single Biomarker Approach

P-tau217 performs best as a standalone test, achieving AUC of 0.928 when combined with cognitive testing (MMSE) for detecting amyloid-positive MCI and dementia 6.

Panel Approach (Superior Performance)

Combining multiple biomarkers yields better diagnostic accuracy than individual markers:

• Optimal panel including all five biomarkers (p-tau217, p-tau181, Aβ42/40, GFAP, NfL) achieves AUC of 0.939 6
• Adding APOE genotype to biomarker combinations further improves accuracy (AUC up to 0.92) 3
• Panel composition should be adjusted for race/ethnicity: p-tau217 for non-Hispanic White patients; p-tau181 for non-White populations 6

Clinical Integration Algorithm

1. Establish objective cognitive impairment through validated cognitive testing and clinical evaluation 4

2. Assess pretest probability of AD pathology based on:

   • Pattern of cognitive deficits (amnestic vs. non-amnestic presentation)
   • Presence of functional impairment
   • Absence of features suggesting non-AD dementia (Lewy body, frontotemporal, vascular) 4

3. Order appropriate blood biomarker panel in specialized care setting:

   • Minimum: p-tau217 or p-tau181
   • Optimal: Multi-biomarker panel including p-tau, Aβ42/40, GFAP, NfL
   • Consider APOE genotyping if available 6, 3

4. Interpret results within clinical context 1:

   • Positive result (above cutoff): Confirms AD pathology with high probability
   • Negative result (below cutoff): Rules out AD pathology with high certainty
   • Intermediate values: Consider additional testing (CSF or PET)

Critical Caveats and Pitfalls

Assay Variability

Many assays remain in development and optimization phases, with performance evolving over time 1. Clinicians must verify which specific assay platforms meet the ≥90%/≥90% accuracy threshold.

Not a Standalone Diagnosis

Blood biomarkers detect AD pathology, not dementia itself 7. They correlate more strongly with amyloid PET (less specific for symptomatic AD) than with tau PET (strongly associated with cognitive changes) 7. Clinical diagnosis still requires comprehensive evaluation including history, examination, and functional assessment 1.

Timing Considerations

Blood biomarkers can detect pathological changes 15-20 years before symptom onset 2, meaning positive results in early stages don't predict when or if symptoms will develop.

Access and Equity

Public health systems should facilitate implementation through appropriate funding to ensure equitable access, as these tests should not be limited to those who can afford direct payment 5.

Current Limitations

The field lacks complete standardization across assay platforms 2, 3. Population-specific validation remains incomplete, particularly for diverse racial and ethnic groups 6. More data are needed before recommending use in general neurology or primary care settings 5.