What laboratory tests are recommended for a patient presenting with paresthesia?

Laboratory Testing for Paresthesia

Routine laboratory testing is not recommended for patients presenting with new-onset paresthesia unless specific risk factors or atypical features are identified through history and physical examination. 1

When Laboratory Testing is NOT Indicated

• Do not obtain routine laboratory panels in patients with typical presentations of peripheral nerve symptoms, as this leads to unnecessary costs, false positives, and patient anxiety without changing clinical outcomes 1
• Avoid reflexive ordering of comprehensive metabolic panels, complete blood counts, or inflammatory markers when history and examination suggest straightforward peripheral nerve compression 1

When Targeted Laboratory Testing IS Indicated

Geographic and Exposure-Based Testing

• Order Lyme disease serology (ELISA followed by Western blot if positive) in patients from endemic areas or with recent travel to endemic regions, as Lyme disease causes up to 25% of facial paralysis cases in these populations 1
• This is the single most important laboratory test to consider based on geographic location 1

Metabolic and Systemic Causes (When History Suggests)

• Obtain fasting glucose and hemoglobin A1c if diabetes is suspected, as diabetic neuropathy is the most common cause of distal symmetric polyneuropathy presenting with paresthesias 2
• Check vitamin B12 levels when patients report dietary restrictions, malabsorption history, or symmetric distal paresthesias, as B12 deficiency is a reversible cause 2
• Order serum protein electrophoresis if monoclonal gammopathy is suspected based on age and symptom pattern 2

Red Flag Presentations Requiring Broader Testing

• Obtain complete blood count, comprehensive metabolic panel, and inflammatory markers when paresthesias are acute (within days), rapidly progressive, asymmetric, or associated with motor weakness, as these features suggest Guillain-Barré syndrome or vasculitis requiring urgent intervention 2
• Consider HIV testing, syphilis serology, and other infectious workup when bilateral facial nerve involvement or multiple cranial neuropathies are present, as these are atypical for simple peripheral causes 1

Critical Clinical Pitfalls

• Never diagnose Bell's palsy and avoid laboratory testing if other neurological deficits are present (limb weakness, language changes, diplopia), as these indicate central nervous system pathology requiring immediate imaging, not laboratory work 3, 4
• Do not delay treatment waiting for laboratory results in suspected Guillain-Barré syndrome, as outcomes worsen with treatment delay 5
• Recognize that facial paresthesias with anomia indicate stroke until proven otherwise, requiring immediate neuroimaging rather than laboratory testing 4

Practical Algorithm

Step 1: Determine if paresthesias are unilateral facial, bilateral facial, or extremity-based through focused neurological examination 3

Step 2: If unilateral facial with no other deficits, check only Lyme serology if in endemic area; otherwise no labs needed 1, 3

Step 3: If extremity paresthesias are distal and symmetric, target testing toward diabetes (glucose/A1c) and B12 deficiency 2

Step 4: If acute onset with motor involvement or rapid progression, obtain CBC, CMP, and ESR/CRP while arranging urgent neurology consultation 2

Step 5: If any central features present (language changes, forehead-sparing weakness, other cranial nerves), skip laboratory testing and proceed directly to urgent brain imaging 3, 4