What initial blood tests should be ordered for a 77-year-old patient presenting with burning sensations in their feet and legs?

Blood Tests for Burning Sensations in Feet and Legs in a 77-Year-Old

For a 77-year-old patient with burning sensations in the feet and legs, the initial blood tests should include complete blood count, urinalysis, serum electrolytes (including calcium and magnesium), blood urea nitrogen, serum creatinine, fasting blood glucose, glycohemoglobin (HbA1c), lipid profile, liver function tests, and thyroid-stimulating hormone. 1

Primary Diagnostic Approach

Essential Blood Tests

• Complete blood count (CBC)
• Urinalysis
• Serum electrolytes (including calcium and magnesium)
• Blood urea nitrogen (BUN)
• Serum creatinine
• Fasting blood glucose
• Glycohemoglobin (HbA1c)
• Lipid profile
• Liver function tests (LFTs)
• Thyroid-stimulating hormone (TSH)

These tests are recommended as the initial laboratory evaluation for patients presenting with symptoms that could indicate peripheral neuropathy, which is a common cause of burning sensations in the feet and legs in older adults 1.

Clinical Context and Rationale

Burning sensations in the feet and legs in a 77-year-old patient most commonly suggest peripheral neuropathy, which has several potential causes that need investigation:

1. Diabetes mellitus: The most common cause of peripheral neuropathy in older adults

   • Fasting glucose and HbA1c are crucial to identify undiagnosed or poorly controlled diabetes 1

2. Renal dysfunction:

   • BUN, creatinine, and electrolytes help assess kidney function, which can affect peripheral nerves when impaired 2

3. Thyroid disorders:

   • TSH abnormalities can contribute to neuropathic symptoms 1

4. Nutritional deficiencies:

   • CBC can help identify anemia that might suggest vitamin B12 deficiency
   • Consider adding vitamin B12 and folate levels if CBC suggests macrocytic anemia

5. Inflammatory or immune-mediated conditions:

   • CBC with differential can identify inflammatory markers
   • Consider adding ESR (erythrocyte sedimentation rate) and CRP (C-reactive protein) if systemic inflammation is suspected

Additional Considerations

If the initial blood tests don't reveal a clear cause, consider second-tier testing based on clinical suspicion:

• Vitamin B12 levels
• Folate levels
• Serum protein electrophoresis (to rule out paraproteinemia)
• Antinuclear antibodies
• Rheumatoid factor
• Anti-SSA/SSB antibodies (if Sjögren's syndrome is suspected)

Clinical Assessment Beyond Blood Tests

While focusing on blood tests, it's important to note that a complete evaluation should also include:

• Assessment for loss of protective sensation using the 10-g monofilament test along with at least one other neurological assessment (pinprick, temperature, vibration using 128-Hz tuning fork, or ankle reflexes) 1
• Vascular assessment including pedal pulses examination 1
• Assessment for foot deformities that might contribute to symptoms 1

Common Pitfalls to Avoid

• Pitfall #1: Focusing only on diabetes as the cause. While diabetes is the most common cause of peripheral neuropathy, other metabolic, toxic, and inflammatory conditions must be considered, especially in patients without known diabetes.

• Pitfall #2: Overlooking medication-induced neuropathy. A careful medication history should accompany laboratory testing, as certain medications can cause peripheral neuropathy.

• Pitfall #3: Missing vitamin B12 deficiency, which is common in older adults and can cause burning sensations in extremities.

• Pitfall #4: Failing to consider vascular causes. Peripheral arterial disease can coexist with neuropathy and may require additional vascular studies if blood tests are inconclusive.

By following this systematic approach to blood testing, clinicians can efficiently identify the underlying cause of burning sensations in the feet and legs of elderly patients, leading to appropriate treatment and improved quality of life.