What is the appropriate diagnostic workup for suspected Alzheimer’s disease in an older adult presenting with progressive memory loss and cognitive changes?

Diagnostic Workup for Suspected Alzheimer's Disease

Begin with a structured clinical assessment documenting gradual cognitive decline over months to years, followed by objective cognitive testing, laboratory studies to exclude reversible causes, brain MRI, and—when available—biomarker testing (plasma p-tau217/181, amyloid-beta, or CSF analysis) to confirm AD pathology. 1

Initial Clinical Assessment

Start by establishing three critical features from the patient and a reliable informant 1:

• Gradual onset over months to years (not sudden over hours/days)
• Clear-cut worsening of cognition by report or direct observation
• Pattern of cognitive deficits on history and examination

Identify the Clinical Presentation Pattern

The most common presentation is amnestic (impaired learning and recall of recent information), but watch for non-amnestic variants 1:

• Language-predominant: Word-finding difficulties as the primary deficit
• Visuospatial-predominant: Object agnosia, impaired face recognition, spatial disorientation
• Executive dysfunction-predominant: Impaired reasoning, judgment, and problem-solving

All presentations require deficits in at least one additional cognitive domain beyond the primary one.

Objective Cognitive Testing

Perform office-based cognitive assessment using validated instruments 1:

• Brief screening: Mini-Cog, Montreal Cognitive Assessment (MoCA), or similar tools
• Domains to test: Memory (especially delayed recall), attention, executive function, visuospatial ability, language
• When to refer for neuropsychological testing: Patient/caregiver report concerning symptoms but normal office testing, OR abnormal results with uncertain interpretation due to very high/low education, confounding demographics, or complex clinical profile 1

Laboratory Evaluation to Exclude Reversible Causes

Order baseline studies to rule out other causes of cognitive decline 1, 2:

• Complete blood count
• Comprehensive metabolic panel
• Thyroid function (TSH)
• Vitamin B12 level
• Consider: RPR/VDRL (if risk factors present), HIV testing (in appropriate populations)

Structural Brain Imaging

MRI of the brain is recommended for all patients with recent cognitive decline 2. This serves to:

• Exclude vascular disease (multiple infarcts, severe white matter disease)
• Identify other structural pathology (tumor, subdural hematoma, normal pressure hydrocephalus)
• Assess for patterns of atrophy consistent with AD (hippocampal/medial temporal lobe atrophy)
• Rule out findings suggesting alternative diagnoses (frontotemporal dementia, Lewy body disease)

Biomarker Testing for Diagnostic Confirmation

The 2025 Alzheimer's Association guidelines emphasize biomarker-based diagnosis 1:

Core AD Biomarkers (in order of accessibility):

Plasma biomarkers (increasingly available, minimally invasive):

• p-tau217 (highest accuracy)
• p-tau181, p-tau231
• Amyloid-beta 42/40 ratio
• GFAP (glial activation marker)

CSF analysis (when plasma unavailable or results equivocal):

• Amyloid-beta 42/40 ratio
• Phosphorylated tau (p-tau181, p-tau217)
• Total tau

PET imaging (typically specialist-ordered):

• Amyloid PET
• Tau PET

Interpreting Biomarker Results

High likelihood of AD when BOTH are positive 1:

• Amyloid pathology (A+): Low plasma/CSF Aβ42/40 ratio OR positive amyloid PET
• Tau pathology/neurodegeneration (T+): Elevated plasma/CSF p-tau OR tau PET OR hippocampal atrophy on MRI

Low likelihood of AD: Both amyloid and tau/neurodegeneration markers negative

Intermediate likelihood: Conflicting results (one positive, one negative) or incomplete testing

Recent evidence shows plasma p-tau181 and GFAP achieve AUCs of 0.755-0.852 for detecting AD pathology, with combined models reaching 0.886-0.893 3.

When to Refer to Specialist

Urgent/expedited specialist referral is required for 1:

• Rapidly progressive dementia (developing over weeks to months)
• Early-onset dementia (age <65 years)
• Atypical presentations: Prominent language/behavioral abnormalities, sensory/motor dysfunction of cerebral origin, difficulty distinguishing from delirium
• Uncertain diagnosis after initial workup
• Normal office cognitive testing despite convincing history of decline

Critical Exclusion Criteria

Do NOT diagnose probable AD dementia if any of the following are present 1:

• Substantial cerebrovascular disease (stroke temporally related to cognitive decline, multiple infarcts, severe white matter disease)
• Core features of Lewy body dementia (visual hallucinations, parkinsonism, REM sleep behavior disorder, fluctuating cognition)
• Prominent behavioral variant frontotemporal dementia features (early personality change, disinhibition, apathy)
• Primary progressive aphasia features
• Other active neurological disease or medications substantially affecting cognition

Common Pitfalls

Depression vs. dementia: New-onset depression in older adults can present with cognitive complaints, but depression is also an early symptom of AD itself 1. Don't dismiss cognitive concerns as "just depression"—evaluate both simultaneously.

Delirium superimposed on dementia: Acute changes require urgent evaluation for underlying medical causes, but don't assume all cognitive impairment is reversible delirium 1.

Overreliance on brief screening: A normal Mini-Cog or MMSE doesn't exclude early AD if the history is compelling—proceed to more detailed testing or neuropsychological evaluation 1.