Biomarker Testing for Alzheimer's Disease

Primary Clinical Indications

• The American Academy of Neurology recommends ordering plasma pTau-217 and Aβ42/40 ratio in patients with mild cognitive impairment (MCI) or mild dementia to detect Alzheimer's disease pathology and determine eligibility for anti-amyloid therapies like lecanemab or donanemab 1, 2, 3
• Plasma pTau-217 alone performs exceptionally well in the dementia stage, with accuracy similar to CSF biomarkers and tau-PET imaging for differentiating AD from other dementias 1, 2, 4
• The pTau-217/Aβ42 ratio shows high diagnostic accuracy (AUC 0.920-0.928) for predicting amyloid positivity in patients with MCI or mild dementia 2, 5, 4
• Adding pTau-217 to Aβ42/Aβ40 has particular value in MCI because pTau levels increase with disease progression and Aβ42/Aβ40 performance may be slightly lower in MCI than in cognitively unimpaired individuals 2, 5, 4
• Both pTau-181 and pTau-217 accurately predict future development of AD dementia in MCI patients over 2-6 years 1, 2, 4

Contraindications and Inappropriate Use

• The American College of Physicians recommends against ordering biomarker testing in cognitively unimpaired individuals without symptoms, as anti-amyloid therapies and their prerequisite biomarker testing are explicitly inappropriate for people who are cognitively unimpaired 3
• The American Geriatrics Society recommends against ordering biomarker testing in patients with positive biomarkers but normal cognition, as donanemab and lecanemab are indicated for symptomatic disease (MCI or mild dementia), not for biomarker-positive individuals with normal cognition 3, 6

Practical Implementation Algorithm

• The National Institute on Aging recommends requiring MoCA score ≤25 OR comprehensive neuropsychological battery showing impairment in ≥1 cognitive domain to establish objective cognitive impairment 3
• The American Academy of Neurology recommends documenting CDR score of 0.5 (MCI) or 1.0 (mild dementia) to confirm functional impairment 3
• The Alzheimer's Association recommends confirming functional impairment on ADCS-iADL or similar scale to establish objective cognitive impairment 3

Critical Caveats and Pitfalls

• The American College of Neurology recommends considering assay selection, as not all pTau assays perform equally, and high-performing platforms include Quanterix Simoa, Lumipulse G, IP-MS methods, and emerging NULISA technology 7, 8
• The National Institute of Neurological Disorders and Stroke recommends being aware of preanalytical considerations, as pTau-217 and pTau-217/Aβ42 ratio are robust to preanalytical delays, but Aβ42 and Aβ40 individually show significant impact from preanalytical delays 8
• The American Academy of Neurology recommends considering biological confounders, such as age, creatinine levels, depressive symptoms, comorbidities, and sex, which are associated with various blood biomarkers 5, 9

Differential Diagnosis Support

• The American College of Neurology recommends combining pTau with other accessible tests, such as APOE genotype and brief cognitive tests, to improve predictive accuracy for AD diagnosis 1, 2, 4
• The National Institute on Aging recommends considering alternative diagnoses, such as frontotemporal dementia or corticobasal degeneration, when pTau is normal but other markers are abnormal 1, 2, 4, 9

False Elevations in Plasma P-tau 217

Pre-analytical Sources of False Elevation

• Systematic errors can occur from differences in blood collection timing, which may artificially alter p-tau217 measurements, according to the Alzheimer's and Dementia guidelines 10
• Using lithium heparin plasma instead of EDTA plasma can introduce systematic measurement differences that falsely elevate or alter p-tau217 values, as stated by the Alzheimer's and Dementia guidelines 10
• Blood draw devices and processing parameters during collection can interfere with AD biomarker measurements and should be carefully standardized, as recommended by the Molecular Neurodegeneration guidelines 11
• Improper centrifugation or failure to remove particulates before analysis can affect measurement accuracy, according to the Molecular Neurodegeneration guidelines 11

Analytical Sources of False Elevation

• Random error from assay variability can result in false classification, especially for individuals with biomarker values close to diagnostic cut-offs, as noted by the Alzheimer's and Dementia guidelines 10
• Systematic error related to changing assay conditions can shift measurements and create false elevations, according to the Alzheimer's and Dementia guidelines 10
• Lack of proper calibration curves, quality control performance, or plate adjustments using bridging samples can introduce measurement drift over time, as stated by the Molecular Neurodegeneration guidelines 11
• Different assays return vastly different numerical values for the same analyte due to different calibrators and technical platforms, which can be misinterpreted as elevated when comparing across platforms, according to the Molecular Neurodegeneration guidelines 11

Biological Confounders

• Cerebrovascular disease and cardiovascular disease are the most important biological confounders that may influence p-tau217 measurements and require careful interpretation, as noted by the Alzheimer's and Dementia guidelines 12

Critical Pitfalls to Avoid

• In real-world clinical settings with heterogeneous populations, performance may not match research cohorts where high accuracies were recorded, as community prevalence of AD pathology is much lower, according to the Molecular Neurodegeneration guidelines 11
• False positives are more likely when testing unselected populations versus highly selected research cohorts that had prior CSF or neuroimaging confirmation, as stated by the Molecular Neurodegeneration guidelines 11
• Cognitively intact older adults can have elevated Aβ levels, but not all will progress to dementia due to resilience factors, as noted by the Journal of Internal Medicine guidelines 13 and 14

Quality Control Measures

• Assay performance metrics must be disclosed, including calibration curve R-values, linearity range, limit of detection, repeatability, and intermediate precision, as recommended by the Molecular Neurodegeneration guidelines 11
• Bridging samples or quality control samples should be used to account for batch variation when samples are analyzed continuously rather than in single batches, according to the Molecular Neurodegeneration guidelines 11
• Certified reference materials should be employed to mitigate between-assay bias and improve comparability across platforms, as stated by the Molecular Neurodegeneration guidelines 11

Likelihood of Alzheimer's Disease with Normal P-tau217 and Normal Beta-Amyloid 42/40

Diagnostic Performance of Normal Biomarkers

• When both p-tau217 and beta-amyloid 42/40 are normal, the odds of having Alzheimer's disease pathology are extremely low, with a negative predictive value of 87-92% across primary and secondary care settings 15

Critical Context: Age-Related Considerations

• 20-40% of cognitively normal older adults over age 60 harbor AD pathological markers, yet many never develop dementia due to resilience factors 15
• In symptomatic patients with cognitive complaints, normal biomarkers strongly argue against AD as the primary etiology 15

Alternative Diagnostic Considerations

• Non-AD neurodegenerative diseases, such as frontotemporal dementia, Lewy body disease, or other tauopathies, should be considered when both biomarkers are normal in a symptomatic patient 16, 17
• Vascular cognitive impairment, hippocampal sclerosis, argyrophilic grain disease, or primary age-related tauopathy can cause cognitive complaints without elevated AD biomarkers 15
• Medical comorbidities, such as metabolic, inflammatory, or medication-related cognitive impairment, should also be considered 16, 17

Prognostic Implications

• Normal p-tau217 in patients with mild cognitive impairment strongly weighs against progression to AD dementia over 2-6 years 16, 17

Clinical Algorithm for Normal Biomarkers

• Reassess the clinical phenotype and evaluate for vascular burden, neuropsychiatric symptoms, non-AD neurodegenerative diseases, and reversible causes when encountering normal p-tau217 and Aβ42/40 in a symptomatic patient 15

Biomarker Utility in Alzheimer's Disease Diagnosis

Diagnostic Performance and Prognosis

• The presence of amyloid-positive and tau-positive status moderately increases the 5-year risk of clinical progression to a prodromal stage, with a hazard ratio of 2.79, according to The Lancet Neurology 18
• 20-40% of cognitively normal older adults over age 60 harbor AD pathological markers, yet many never develop dementia due to resilience factors, limiting predictive value in asymptomatic individuals, as reported by The Lancet Neurology 18