What laboratory tests should be ordered to evaluate peripheral neuropathy?

Laboratory Testing for Peripheral Neuropathy

Order a focused screening panel consisting of: blood glucose (or HbA1c), serum B12 with metabolites (methylmalonic acid ± homocysteine), and serum protein immunofixation electrophoresis—these three tests have the highest diagnostic yield. 11

Core Screening Panel

The American Academy of Neurology guidelines establish that while a comprehensive basic panel is often performed, the three highest-yield tests are:

• Blood glucose/HbA1c - Abnormal in ~11% of patients (diabetes is the most common cause of neuropathy)
• Serum B12 with metabolites - Deficiency found in 2.2-8% of patients
• Serum protein immunofixation electrophoresis - Abnormal in ~9% of patients

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Critical Point About B12 Testing

Do not order B12 alone—always include methylmalonic acid (MMA) with or without homocysteine. Standard B12 levels miss 44% of B12-deficient patients with neuropathy. MMA is more specific than homocysteine (which can be elevated in folate deficiency and other conditions). 11

Extended Basic Panel

While the above three tests have the highest yield, a standard comprehensive screening panel typically includes:

• Complete blood count
• Erythrocyte sedimentation rate
• Comprehensive metabolic panel (renal function, liver function)
• Thyroid function tests

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Important caveat: When combined with clinical history, neurological examination, and electrodiagnostic studies, this comprehensive approach yields an etiological diagnosis in 74-82% of patients. However, laboratory tests alone have only 37% diagnostic yield—they must be interpreted in clinical context. 1

Glucose Testing Nuance

If routine blood glucose is not clearly abnormal but you suspect diabetic neuropathy (especially with painful distal symmetric sensory neuropathy), consider glucose tolerance testing (GTT) to detect pre-diabetes/impaired glucose tolerance. 11

When to Order Additional Tests

Do not order additional specialized tests routinely. Further testing should be guided by:

• Specific clinical phenotype (demyelinating vs. axonal, acute vs. chronic, symmetric vs. asymmetric)
• Unexpected disease course changes
• Failure of basic screening to identify etiology

In these scenarios, refer to a neuromuscular center. 23

Genetic Testing Considerations

Genetic testing is established as accurate for hereditary neuropathies (100% specificity) but should only be ordered when patients exhibit a hereditary neuropathy phenotype (family history, early onset, pes cavus, etc.). Initial testing should focus on: CMT1A duplication/HNPP deletion, Cx32 (GJB1), and MFN2 mutations. 11

There is insufficient evidence to support routine genetic testing in cryptogenic neuropathy without hereditary features. 1

Common Pitfalls to Avoid

• Ordering B12 without metabolites - Misses nearly half of deficient patients
• Using serum protein electrophoresis (SPEP) instead of immunofixation (IFE) - IFE detects 30% more IgM monoclonal gammopathies
• Ordering extensive panels in all patients - Leads to false positives and unnecessary costs; tailor testing to clinical presentation
• Skipping electrodiagnostic studies - These are essential for phenotyping and should precede or accompany laboratory workup

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