What blood tests are recommended for numb feet due to suspected nerve dysfunction?

Blood Tests for Numb Feet Due to Suspected Nerve Dysfunction

When evaluating numb feet due to suspected nerve dysfunction, blood tests should focus on identifying common causes of neuropathy other than diabetes, including vitamin B12 deficiency, hypothyroidism, renal disease, and toxins. 1

Recommended Blood Tests

Based on the most recent guidelines, the following blood tests should be ordered when evaluating numb feet with suspected nerve dysfunction:

1. Glucose metabolism tests:

   • Fasting blood glucose
   • HbA1c
   • Glucose tolerance test (particularly valuable but underutilized) 2

2. Vitamin deficiencies:

   • Vitamin B12 levels 1
   • Serum folate 3

3. Endocrine function:

   • Thyroid function tests (TSH, free T4) 1, 3

4. Renal function:

   • Blood urea nitrogen (BUN)
   • Serum creatinine
   • eGFR 1

5. Other relevant tests:

   • Complete blood count
   • Liver function tests
   • Electrolytes 3

Clinical Context and Rationale

Diabetic peripheral neuropathy (DPN) is a common cause of numb feet, but it's essential to rule out other treatable causes. According to the American Diabetes Association guidelines, in all patients with neuropathy, causes other than diabetes should be considered, including:

• Toxins (e.g., alcohol)
• Neurotoxic medications (e.g., chemotherapy)
• Vitamin B12 deficiency
• Hypothyroidism
• Renal disease
• Malignancies (e.g., multiple myeloma, bronchogenic carcinoma)
• Infections (e.g., HIV)
• Chronic inflammatory demyelinating neuropathy
• Inherited neuropathies
• Vasculitis 1

Important Clinical Considerations

• Diabetic neuropathy is a diagnosis of exclusion 1
• Up to 50% of diabetic peripheral neuropathy may be asymptomatic 1
• Glucose tolerance testing is particularly valuable but significantly underutilized (only 1% of patients receive this test despite its high yield) 2
• Prediabetes is associated with similar risks of peripheral neuropathy as new-onset diabetes, highlighting the importance of glucose metabolism testing even in patients without diagnosed diabetes 4

Common Pitfalls to Avoid

1. Overreliance on expensive imaging: Nearly 25% of patients undergo high-cost, low-yield MRI studies of the brain or spine, while few receive appropriate metabolic testing 2

2. Failure to consider prediabetes: Prediabetes is independently associated with both peripheral neuropathy and nerve dysfunction, making glucose tolerance testing particularly important 4

3. Excessive testing without a systematic approach: Studies show highly variable testing patterns with a median of 4 tests performed per patient, but with over 400 different patterns of testing observed 2

4. Missing treatable causes: Failing to test for vitamin B12 deficiency, hypothyroidism, or renal disease may result in missing easily treatable causes of neuropathy 1

By following this systematic approach to blood testing for numb feet, clinicians can efficiently identify the underlying cause of nerve dysfunction and implement appropriate treatment strategies to improve outcomes and quality of life.