What is the appropriate workup for cognitive impairment?

Workup for Cognitive Impairment

The workup for cognitive impairment requires a structured three-step approach: (1) clinical assessment with patient and informant history plus objective cognitive testing to establish the presence and pattern of impairment, (2) laboratory and imaging studies to identify reversible causes, and (3) biomarker testing (CSF or PET) when Alzheimer's disease is suspected and the diagnosis will impact management decisions.

Step 1: Clinical Assessment

History and Cognitive Testing

Begin by obtaining history from both the patient AND a knowledgeable informant—this is non-negotiable 1. You need to establish:

• Cognitive concern: Document whether the patient, informant, or you as the clinician have observed decline over time
• Functional impact: Determine if the impairment interferes with independence in everyday activities (distinguishes MCI from dementia)
• Cognitive domains affected: Assess at minimum these domains 1:
  • Memory (acquisition and recall of new information)
  • Executive function (reasoning, complex tasks, judgment)
  • Visuospatial abilities
  • Language functions
  • Personality/behavioral changes

Perform objective cognitive testing—a "bedside" mental status examination is acceptable, but neuropsychological testing should be done when bedside testing cannot provide confident diagnosis 1. The pattern of impairment guides your differential diagnosis 2.

Distinguish MCI from Dementia

• MCI criteria: Objective cognitive impairment with preserved independence in functional abilities (though may require greater effort or compensatory strategies) 1
• Dementia criteria: Cognitive/behavioral symptoms that interfere with ability to function at work or usual activities AND represent decline from previous levels 1

Step 2: Laboratory and Imaging Workup

Conduct a thorough medication review to identify drugs with anticholinergic burden or sedating properties that affect cognition 3, 4.

Order laboratory studies to identify reversible causes of cognitive impairment 4. While the guidelines don't specify exact tests, standard practice includes thyroid function, B12, metabolic panel, and consideration of other causes based on clinical presentation.

Obtain brain imaging to evaluate for structural causes 4. This helps identify vascular disease, space-occupying lesions, and patterns of atrophy.

Step 3: Biomarker Testing (When Appropriate)

When to Order Biomarkers

Biomarker testing is indicated when you suspect Alzheimer's disease based on the clinical phenotype (typically amnestic presentation of hippocampal type or other common AD phenotypes) 5.

Critical Caveat About Biomarkers

Do NOT order biomarker testing in cognitively unimpaired individuals in routine clinical practice 5. The current inability to predict reliable clinical trajectories makes this inappropriate outside of research settings or when patients are being considered for disease-modifying trials.

Which Biomarkers to Order

Prioritize CSF testing over PET imaging because it simultaneously provides information on both amyloid and tau pathology and is less expensive 5. Order lumbar puncture for:

• CSF Aβ40–Aβ42 ratio (preferred over Aβ42 alone)
• CSF phosphorylated tau (NOT total tau due to low specificity)

If lumbar puncture is contraindicated, use PET imaging as an alternative 5.

Interpreting Biomarkers

Both amyloid AND tau biomarkers must be positive to diagnose Alzheimer's disease 5. This is crucial because:

• Amyloid positivity alone occurs in other conditions (LATE, dementia with Lewy bodies, cerebral amyloid angiopathy)
• Tau positivity alone can occur in primary age-related tauopathy or atypical frontotemporal lobar degeneration

Important: Plasma biomarkers for amyloid and tau are NOT currently recommended for clinical practice—they require further standardization and validation 5.

When Results Are Borderline

If cognitive testing or biomarker results are close to cutoff points, complete the workup with repeated measures, clinical follow-up, or neurodegeneration biomarkers like FDG-PET 5.

Common Pitfalls to Avoid

1. Failing to obtain informant history: Patient self-report alone is insufficient, particularly in cognitive impairment where insight may be limited
2. Ordering biomarkers in asymptomatic individuals: This is not recommended in routine practice 5
3. Relying on amyloid biomarkers alone: Both amyloid AND tau positivity are required for AD diagnosis 5
4. Using total tau instead of phosphorylated tau: Total tau lacks specificity 5
5. Neglecting medication review: Anticholinergic burden and sedating medications are modifiable contributors to cognitive impairment 3

Management Considerations After Diagnosis

Once MCI is identified, the mainstay of intervention involves a multi-domain approach: optimizing metabolic health, mitigating cerebrovascular risk, correcting hearing and vision loss, treating depression, encouraging cognitive exercises and social connection, and avoiding harmful substances 3. Address cardiovascular risk factors aggressively and reduce cholinergic burden 3.