Alzheimer's disease is no longer defined purely by symptoms; it is a biological disease that can now be detected during life. Blood-based biomarkers (BBMs) allow primary care physicians to move from uncertainty to structured triage and early direction.
The goal is not to diagnose Alzheimer's in primary care. The goal is to determine probability - decide direction - expedite the right pathway.
1. The Clinical Shift: From Symptoms to Biology
In patients with objective cognitive impairment, blood biomarkers function as probability tools, not standalone diagnoses.
Negative test - AD unlikely - pivot diagnosis
Positive test - AD likely - refer and confirm
Indeterminate - early disease or noise - monitor or escalate
This aligns with what we already know clinically: cognitive impairment is a syndrome; biomarkers help define the cause.
2. Who Should Be Tested (and Who Should Not)
Appropriate Patients
Test only when there is objective cognitive impairment:
Mild Cognitive Impairment (MCI)
Mild dementia
MCI represents measurable decline with preserved independence and carries risk of progression.
Do NOT Test
Asymptomatic patients
Subjective complaints with normal testing
Routine screening or family history alone
Key rule: No syndrome - no biomarker
3. Before You Order: Establish the Clinical Frame
Biomarkers should never be first-line.
Complete the baseline workup:
Cognitive screen (MoCA, MMSE, SLUMS)
Functional assessment (independence vs impairment)
Medication review
Reversible labs (B12, TSH, CMP)
Sleep, mood, and systemic contributors
This step prevents false attribution of biology to reversible disease.
4. What to Order (Keep It Simple)
Core Strategy
1. Rule-Out Tool - p-tau181
Best for excluding Alzheimer’s biology
High negative predictive value (~97-98%)
If negative - look elsewhere
2. Rule-In / Probability Tool - p-tau217 (± Aβ42/40)
Most accurate blood marker for AD biology
Positive - high likelihood of amyloid + tau pathology
Best Practical Approach
If available, use a combined panel:
p-tau217 + Aβ42/40
Why:
Gives a probability score, not just a binary result
More clinically actionable than either marker alone
5. Interpreting Results (What You Actually Do Next)
Negative p-tau
Meaning: Alzheimer’s biology unlikely
Action:
Stop AD pathway
Reframe diagnosis:
Vascular disease
Lewy body disease
Frontotemporal degeneration
Mood, sleep, medications
Dementia is often non-Alzheimer's or mixed, not singular.
Positive p-tau
Meaning: Alzheimer’s biology likely present
Action:
Confirm clinical stage (MCI vs dementia)
Refer to neurology / memory clinic
Initiate treatment pathway discussion
Next step is confirmation, not repetition:
Amyloid PET or
CSF biomarkers
This is required before anti-amyloid therapy.
Intermediate / Borderline
Meaning: Early disease or assay uncertainty
Action:
Repeat in 6-12 months
OR refer for definitive testing
6. What Changes with Blood Testing
Adding BBMs to clinical evaluation:
Improves diagnostic accuracy from ~60% - ~90%
Reduces diagnostic delay dramatically
Allows earlier access to disease-modifying therapy pathways
This is not about certainty; it is about reducing diagnostic drift.
7. Critical Caveats (High-Yield)
Biologic ≠ Clinical
A positive biomarker does not equal dementia - Always interpret in clinical context.
Confounders Matter
CKD - false positives (reduced clearance)
Obesity - false negatives (dilution effect)
Mixed Pathology is Common
Most patients are not "pure Alzheimer's"
Expect overlap:
Alzheimer's + vascular
Alzheimer's + Lewy body
Biomarkers detect one component, not the whole disease.
Insurance & Legal Reality
APOE is protected under GINA
Protein biomarkers are not
This has implications for:
Life insurance
Disability
Long-term care
8. The PCP Role (Bottom Line)
You are not being asked to diagnose Alzheimer’s disease. You are being asked to:
Identify cognitive impairment
Apply a biologic probability tool
Direct the patient into the correct pathway