What is the next step in bloodwork for a patient with peripheral neuropathy, lower extremity pain, and decreased pinprick sensation, who has a history of alcohol abuse, diabetes mellitus type 2, and labs showing elevated Mean Corpuscular Volume (MCV) and hyperglycemia (Hemoglobin A1c 8.1%)?

Next Step in Bloodwork: Order Serum Vitamin B12

Order serum vitamin B12 immediately – this patient's elevated MCV (macrocytosis) combined with peripheral neuropathy in the setting of alcohol abuse strongly suggests B12 deficiency, which is a treatable cause that must be excluded before attributing all symptoms to diabetic neuropathy alone 1, 2, 3.

Clinical Reasoning for B12 Testing

Why B12 is the Priority

• Elevated MCV is the key diagnostic clue: Macrocytosis points toward either B12 deficiency or folate deficiency, both of which can cause peripheral neuropathy 3
• Alcohol abuse significantly increases B12 deficiency risk: Chronic alcohol use impairs B12 absorption and storage, making deficiency highly prevalent in this population 2, 3
• B12 deficiency causes peripheral neuropathy independently: The neuropathy may be partially or entirely due to B12 deficiency rather than diabetes, and B12 neuropathy is reversible if caught early 1, 2
• Diabetic neuropathy is a diagnosis of exclusion: Before confirming diabetic peripheral neuropathy, alternative causes including B12 deficiency must be ruled out 1, 2, 3

Why Not the Other Options

Thiamine (option b) is less likely despite alcohol abuse because:

• Thiamine deficiency typically presents with Wernicke-Korsakoff syndrome (confusion, ataxia, ophthalmoplegia) or beriberi (heart failure, edema) rather than isolated peripheral neuropathy 2
• Thiamine deficiency does not cause elevated MCV 3
• While thiamine should eventually be checked in alcoholic patients, the elevated MCV makes B12 the more urgent priority

Lower extremity vascular studies (option c) are not the next step because:

• Vascular studies assess for peripheral arterial disease, not neuropathy 4
• The patient's symptoms (decreased pinprick sensation, peripheral neuropathy) indicate nerve dysfunction, not vascular insufficiency 4, 1
• Vascular studies would be indicated if there were signs of critical limb ischemia (rest pain, dependent rubor, absent pulses, ulcers) 4

EMG/nerve conduction studies (option d) are premature because:

• Electrodiagnostic studies are rarely needed except when clinical features are atypical (asymmetric distribution, rapid progression, motor predominance, upper extremity involvement) 1, 2
• The American Diabetes Association emphasizes that diabetic peripheral neuropathy is diagnosed through clinical examination, not electrodiagnostic testing 1
• Laboratory exclusion of treatable causes (B12 deficiency) must come before expensive electrodiagnostic studies 1, 3

Complete Initial Laboratory Workup

Beyond B12, the comprehensive initial evaluation for peripheral neuropathy in this patient should include 1, 2, 3:

• Vitamin B12 level (primary concern given elevated MCV)
• Complete metabolic panel (CMP) to assess renal function, as uremia can cause neuropathy 1, 2
• Thyroid-stimulating hormone (TSH) to exclude hypothyroidism 1, 3
• Complete blood count (already done, showing elevated MCV)
• Hemoglobin A1c (already done at 8.1%, confirming poor glycemic control)

Critical Clinical Pitfalls to Avoid

• Never assume all neuropathy in diabetics is diabetic neuropathy: Up to 10% of neuropathy in diabetic patients has alternative or contributing causes 3
• Do not overlook B12 deficiency in alcoholics: The combination of alcohol abuse and macrocytosis makes B12 deficiency highly likely 2, 3
• Do not delay B12 testing: B12 neuropathy can become irreversible if treatment is delayed beyond 6-12 months 5
• Recognize that multiple causes can coexist: This patient likely has both diabetic neuropathy (HbA1c 8.1%) and B12 deficiency (elevated MCV, alcohol abuse), requiring treatment of both 2, 3

Management After B12 Results

If B12 is low (<200 pg/mL) 5:

• Start intramuscular B12 100 mcg daily for 6-7 days, then alternate days for 7 doses, then every 3-4 days for 2-3 weeks, followed by 100 mcg monthly for life 5
• Simultaneously optimize glycemic control to address diabetic neuropathy component 2
• Consider first-line neuropathic pain medication (pregabalin, duloxetine, or gabapentin) if pain persists 2

If B12 is normal:

• Proceed with TSH and CMP to exclude other treatable causes 1, 3
• Attribute neuropathy primarily to diabetes and alcohol, focusing on glycemic optimization and alcohol cessation 2