What labs should be ordered for a patient presenting with hyperreflexia?

Laboratory Evaluation for Hyperreflexia

For patients presenting with hyperreflexia, a comprehensive laboratory evaluation should include complete blood count, urinalysis, serum electrolytes, blood urea nitrogen, serum creatinine, glucose, liver function tests, thyroid-stimulating hormone, serum B12 with metabolites (methylmalonic acid and homocysteine), and serum protein immunofixation electrophoresis to identify the underlying cause. 1, 2

Primary Laboratory Tests

The initial laboratory workup for hyperreflexia should include:

• Complete blood count (CBC) - To evaluate for anemia, infection, or other hematologic abnormalities
• Comprehensive metabolic panel:
  • Blood glucose (highest diagnostic yield in polyneuropathy evaluation) 1
  • Renal function tests (BUN, creatinine)
  • Liver function tests
  • Serum electrolytes including calcium and magnesium
• Thyroid function tests - Particularly thyroid-stimulating hormone (TSH) 1, 2
• Serum B12 with metabolites - Including methylmalonic acid and homocysteine (higher sensitivity than B12 alone) 1
  • Methylmalonic acid is elevated in 98.4% of B12 deficiency cases
  • Homocysteine is elevated in 95.9% of B12 deficiency cases but is less specific
• Serum protein immunofixation electrophoresis - To detect paraproteinemias 1

Additional Testing Based on Clinical Suspicion

Depending on the clinical presentation, additional tests may include:

• Iron studies - To screen for hemochromatosis, particularly in patients of Northern European descent 1
• HIV screening - Particularly in high-risk patients 1
• Electrocardiogram (ECG) - To assess for cardiac etiology 2
• Chest radiograph - To evaluate for cardiopulmonary pathology 2

Diagnostic Considerations

When evaluating hyperreflexia, it's important to note:

1. Highest yield tests: Studies indicate that blood glucose, serum B12, and serum protein immunofixation electrophoresis have the highest diagnostic yield in patients with neurological symptoms 1

2. B12 deficiency detection: Serum B12 assays with metabolites (methylmalonic acid and homocysteine) are more sensitive than B12 alone in documenting deficiency. Approximately 44% of patients with polyneuropathy and B12 deficiency may have normal B12 levels but abnormal metabolites 1

3. Cerebrospinal fluid analysis: Generally has low diagnostic yield except in demyelinating conditions, which typically show increased CSF protein levels 1

Clinical Pitfalls to Avoid

• Don't rely solely on serum B12 levels: Testing metabolites (methylmalonic acid and homocysteine) increases diagnostic yield for B12 deficiency 1

• Don't overlook thyroid dysfunction: Both hyperthyroidism and hypothyroidism can contribute to neurological symptoms 1

• Don't miss potential infectious causes: Consider screening for infectious etiologies in appropriate clinical contexts 1

• Avoid interpreting laboratory results in isolation: Laboratory test results must be interpreted in the context of other clinical information, as many tests have limited specificity when used alone 1