What laboratory tests should be ordered to screen for infection, inflammatory disease, diabetes‑related neuropathy, vitamin D and B12 deficiencies, rheumatologic disease, and possible Lyme disease?

Laboratory Workup for Distal Symmetric Polyneuropathy

For screening polyneuropathy, order a comprehensive metabolic panel (glucose, renal/liver function), complete blood count, ESR, serum B12 with methylmalonic acid, thyroid function tests (TSH), and serum protein immunofixation electrophoresis as your core battery 1.

Core Screening Tests (Order for All Patients)

The 2009 AAN Practice Parameter establishes that these basic tests yield the highest diagnostic return, identifying an etiology in 74-82% of polyneuropathy cases when combined with clinical and electrodiagnostic findings 1:

• Blood glucose and HbA1c - Highest yield test (~11% abnormal); diabetes is the most common cause of polyneuropathy 1
• Serum B12 with metabolites - Critical addition: order methylmalonic acid and homocysteine alongside B12, as 44% of B12-deficient patients have normal B12 levels but elevated metabolites 1
• Serum protein immunofixation electrophoresis - Abnormal in ~9% of cases 1
• Complete blood count - Standard screening 1, 2
• Comprehensive metabolic panel - Captures renal function (creatinine) and liver function (ALT, AST) 1, 2
• ESR - Screens for inflammatory conditions 1, 2
• TSH and free T4 - Hypothyroidism is a treatable cause 1, 2

Critical Pitfall: B12 Testing

Do not rely on serum B12 alone. When B12 is in the low-normal range (200-500 pg/dL), methylmalonic acid and homocysteine are elevated in 5-10% of patients 1. Methylmalonic acid is more specific than homocysteine (which also elevates in folate deficiency), with 98.4% sensitivity for B12 deficiency 1.

Vitamin D Testing

Do not routinely order vitamin D for polyneuropathy workup. While vitamin D deficiency is prevalent in inflammatory rheumatic diseases, current evidence does not support a causal or disease-modifying role in neuropathy 3. Reserve vitamin D testing for patients with concurrent rheumatologic symptoms or skeletal health concerns.

Lyme Disease Testing - When and How

Only test for Lyme disease in patients with specific neurologic presentations AND epidemiologically plausible tick exposure 4:

Test for Lyme if patient has:

• Acute meningitis
• Painful radiculoneuritis
• Mononeuropathy multiplex
• Acute cranial neuropathies (especially CN VII, VIII)
• Spinal cord inflammation with radiculitis
• AND plausible exposure to Ixodes ticks in endemic areas 4

Do NOT test for Lyme in:

• Typical ALS, relapsing-remitting MS, Parkinson's disease
• Dementia or cognitive decline
• New-onset seizures
• Psychiatric illness
• Nonspecific brain MRI white matter changes
• Absence of clinical/epidemiologic support 4

Use serum antibody testing, not PCR or culture 4. For erythema migrans with typical presentation in endemic areas, diagnose clinically without laboratory testing 4.

Rheumatologic Screening

The basic battery already captures inflammatory markers (ESR) and protein abnormalities (immunofixation). Expand rheumatologic workup only if:

• ESR is elevated
• Clinical signs suggest systemic autoimmune disease (arthritis, sicca symptoms, rash)
• Subacute onset or asymmetric presentation
• Demyelinating pattern on electrodiagnostic studies 5

Then add: ANA, RF, anti-SSA/SSB (for Sjögren's), ANCA if vasculitis suspected.

What NOT to Order Routinely

• Paraneoplastic panels - Only 35-52% of neurologists order these for secondary evaluation; reserve for rapidly progressive, severe, or atypical presentations 6
• Vitamin D - Not part of standard neuropathy evaluation 3
• CSF analysis - Low yield except in demyelinating polyneuropathies (shows elevated protein) 1
• Lyme testing - Without appropriate clinical syndrome and exposure history 4

Algorithm Summary

1. All patients: CBC, CMP, ESR, glucose/HbA1c, B12 + methylmalonic acid + homocysteine, TSH, serum protein immunofixation
2. If basic tests negative + specific clinical features: Targeted testing based on presentation (asymmetric → vasculitis workup; demyelinating → consider CSF; endemic area + acute radiculoneuritis → Lyme serology)
3. Refer to neuromuscular specialist if basic workup unrevealing and presentation is atypical, rapidly progressive, or involves motor weakness 7

This approach balances comprehensive screening for treatable causes while avoiding low-yield, expensive testing that contributes to diagnostic uncertainty.