What blood tests should be ordered to assess amyloid burden in an older adult with suspected dementia or mild cognitive impairment?

Blood-Based Biomarker Testing for Amyloid Pathology in Suspected Dementia

Order blood tests measuring plasma phosphorylated tau-217 (p-tau217) or the ratio of p-tau217 to non-p-tau217 (%p-tau217), ideally combined with plasma Aβ42/Aβ40 ratio (such as the Amyloid Probability Score 2), as these achieve ≥90% sensitivity and specificity for detecting amyloid pathology in older adults with cognitive impairment. 1, 2

Primary Recommended Blood Biomarkers

The highest-performing blood tests for assessing amyloid burden include:

• P-tau217 or %p-tau217: These demonstrate AUCs of 0.92-0.98 for detecting amyloid pathology, approaching the performance of CSF testing 3
• Combined algorithms (e.g., APS2): Incorporating plasma Aβ42/Aβ40 ratio with %p-tau217 achieves 90-91% accuracy, 88-91% PPV, and 87-92% NPV in both primary and secondary care settings 4, 5
• P-tau181 and p-tau231: Alternative phosphorylated tau markers, though generally with slightly lower performance than p-tau217 3

Performance Thresholds for Clinical Use

For Triaging (Ruling Out Amyloid Pathology)

Blood tests should meet minimum criteria of ≥90% sensitivity and ≥75% specificity to serve as initial screening tools 1, 2. A negative result on such tests effectively rules out amyloid pathology in most patients, reducing unnecessary referrals for confirmatory testing 3.

For Confirmation (Replacing CSF/PET)

Blood tests must achieve ≥90% sensitivity AND ≥90% specificity to serve as standalone confirmatory tests for amyloid pathology 1, 2. Tests meeting these thresholds can substitute for amyloid PET or CSF biomarker testing in the diagnostic workup 1.

Clinical Implementation Pathway

Patient Selection Criteria

Blood biomarker testing is appropriate for:

• Adults with objective cognitive impairment (not just subjective complaints) documented by cognitive testing 3
• Age typically >55 years in primary care or >65 years for confirmation testing 3
• Progressive cognitive decline, preferably corroborated by an informant 3
• After ruling out other reversible causes of cognitive impairment 3

Interpretation Framework Based on Clinical Setting

In Primary Care (Triaging Approach):

• Positive blood test (meeting ≥90% sensitivity/≥85% specificity): Indicates high likelihood of amyloid pathology but requires confirmatory testing with CSF or amyloid PET before initiating disease-modifying treatments 3
• Negative blood test: Rules out amyloid pathology in most cases, avoiding unnecessary specialist referral 3

In Secondary Care (Confirmation Approach):

• Blood tests achieving ≥90% sensitivity and ≥90% specificity can confirm amyloid pathology when interpreted alongside comprehensive clinical assessment 1, 2
• This approach improves diagnostic accuracy from 61-73% (clinical assessment alone) to 90-91% when incorporating blood biomarkers 4

Critical Performance Considerations

Test-Specific Variability

Not all commercially available blood tests meet recommended performance thresholds 1, 2. Diagnostic accuracy varies dramatically by:

• Assay platform (mass spectrometry typically outperforms immunoassays) 3
• Specific antibodies and reagents used 3
• Laboratory processing and storage conditions 3

Plasma Aβ42/Aβ40 ratio alone has AUCs of only 0.70-0.85, insufficient for standalone diagnostic use 3. In contrast, p-tau217-based tests achieve AUCs of 0.92-0.98 3.

Pretest Probability Matters

Blood biomarker performance depends heavily on clinical context 3, 2:

• In populations with high clinical suspicion (60-80% prevalence of amyloid pathology), positive tests have PPVs of 88-91% 3, 4
• In populations with low clinical suspicion (20% prevalence), PPVs drop to 60-69%, necessitating confirmatory testing 3

Integration with Disease-Modifying Treatments

When DMTs Are Being Considered

• CSF or amyloid PET remains preferable if disease-modifying treatments like lecanemab or donanemab are under consideration 3
• However, blood tests meeting ≥90% sensitivity and specificity thresholds are acceptable for confirming amyloid pathology required for DMT eligibility 3
• Lecanemab prescribing information does not specify the biomarker modality, leaving blood tests as a viable option 3

Common Pitfalls to Avoid

Do Not Use Blood Tests As:

• Standalone diagnostic tools without comprehensive clinical evaluation 1, 2
• Screening tests in asymptomatic individuals or those with only subjective cognitive complaints 3
• Substitutes for ruling out other dementia etiologies (vascular, frontotemporal, Lewy body) 3

Recognize Limitations:

• Blood tests detect amyloid pathology but do not confirm clinical Alzheimer's disease diagnosis alone 3, 1
• Up to 30% of cognitively normal older adults have amyloid positivity without clinical symptoms 3
• Patients on anticoagulation who cannot undergo lumbar puncture may particularly benefit from blood testing 3

Laboratory Considerations:

• Ensure the specific test used meets performance thresholds—brand and platform matter significantly 1, 2
• Results should be interpreted only by providers trained in dementia evaluation 3, 1
• Blood collection and handling protocols must be strictly followed to ensure accuracy 3

Practical Advantages Over Traditional Testing

Blood biomarkers offer substantial benefits:

• More acceptable to patients than lumbar puncture 3
• More accessible and scalable than amyloid PET imaging 3
• Lower cost than neuroimaging or CSF analysis 3
• Faster turnaround with biweekly processing maintaining 90%+ accuracy 4
• Can be performed in primary care settings, addressing the shortage of dementia specialists 3