Numbness & Tingling: Which Labs to Check for Peripheral Neuropathy

Overview.

Numbness and tingling (paraesthesia) in the hands or feet is the hallmark symptom of peripheral neuropathy — damage to the nerves outside the brain and spinal cord. The two most common causes are diabetes (even prediabetes) and vitamin B12 deficiency, both readily detectable with blood tests. Approximately 10% of the general population and 50% of diabetics have peripheral neuropathy. Early detection prevents irreversible nerve damage.

Peripheral neuropathy presents as numbness, tingling, burning, or 'pins and needles' — typically starting in the feet and progressing proximally (glove-and-stocking pattern). It can be sensory (numbness), motor (weakness), or autonomic (blood pressure, sweating, GI). The clinical priority is identifying the cause — many are treatable if caught early.

Prevalence: ~10% of general population. ~50% of diabetics. B12 deficiency neuropathy: ~2-5% of adults over 60. Up to 30% of neuropathy cases are 'idiopathic' (no identified cause), but many of these have undetected glucose intolerance or subclinical B12 deficiency.

What to test.

First-line tests

• Vitamin B12 — B12 deficiency is the most common nutritional cause of neuropathy. It damages myelin (nerve insulation), causing numbness, tingling, and eventually irreversible nerve damage. Affects vegetarians, vegans, metformin users, and older adults. Neuropathy can occur at B12 200-400 pg/mL ('low-normal').
• HbA1c / Fasting Glucose — Diabetic neuropathy is the most common cause overall. BUT: neuropathy can occur in prediabetes (HbA1c 5.7-6.4%) — post-prandial glucose spikes damage nerves even when fasting glucose is normal. HbA1c catches what fasting glucose misses.
• TSH — Hypothyroidism causes carpal tunnel syndrome (median nerve compression) and generalised peripheral neuropathy through myxoedematous changes. Easily treatable once identified.
• CBC — Macrocytic anaemia (high MCV >100 fL) raises suspicion for B12 or folate deficiency. However, neuropathy can occur without anaemia — normal MCV does not rule out B12 deficiency.

Second-line tests

• Folate — Folate deficiency causes neuropathy through the same methylation pathway as B12. Check alongside B12. Note: folate supplementation can mask B12 deficiency (corrects anaemia but neuropathy progresses).
• Vitamin D — Vitamin D deficiency is associated with small fibre neuropathy (burning/tingling without loss of reflexes). Receptors present on nerve cells. Repletion may improve neuropathic symptoms.
• CMP — eGFR/creatinine for uraemic neuropathy (CKD). Liver function for hepatic neuropathy. Calcium for hypocalcaemic paraesthesia.
• Fasting Insulin / HOMA-IR — If HbA1c is 'normal' but prediabetic range (5.4-5.6%), fasting insulin may reveal insulin resistance causing post-prandial glucose spikes that damage nerves before HbA1c crosses the threshold.

Specialized tests

• Methylmalonic Acid (MMA) — The definitive test for tissue-level B12 deficiency. Elevated MMA confirms functional B12 deficiency even when serum B12 is in the 'low-normal' range (200-400 pg/mL). Order when B12 is borderline and neuropathy is present.
• SPEP / Immunofixation — Screens for monoclonal gammopathy (MGUS, myeloma) — a cause of neuropathy in patients >60 with no other explanation. Paraprotein-associated neuropathy affects ~5% of idiopathic neuropathy cases.

Common causes.

• Diabetes / Prediabetes — Symmetric distal sensory neuropathy (feet first, then hands). Burning, tingling, numbness in a stocking-glove pattern.
• Vitamin B12 Deficiency — Numbness and tingling in hands and feet, balance problems (posterior column involvement), cognitive changes
• Hypothyroidism — Carpal tunnel syndrome (median nerve — thumb, index, middle finger numbness) or generalised neuropathy

Diagnostic patterns.

• Symmetric foot numbness + HbA1c 5.8% + elevated fasting insulin — likely Prediabetic neuropathy. Next step: Aggressive glucose management: low-glycaemic diet, exercise, weight loss; consider metformin
• Hand and foot numbness + B12 250 pg/mL + elevated MMA + vegan diet — likely Functional B12 deficiency. Next step: B12 injections (1000μg IM weekly x 4-8 weeks, then monthly); neuropathy improves if caught early
• Thumb/index numbness + elevated TSH + positive Tinel's/Phalen's test — likely Hypothyroid carpal tunnel syndrome. Next step: Levothyroxine; carpal tunnel often resolves within months of thyroid treatment
• Neuropathy + age >60 + monoclonal protein on SPEP — likely Paraprotein-associated neuropathy (MGUS/myeloma). Next step: Haematology referral; bone marrow biopsy if myeloma suspected

Lifestyle.

Non-medical causes

• Positional compression (sitting on foot, sleeping on arm — resolves in minutes)
• Carpal tunnel from repetitive strain (typing, manual work)
• Anxiety/hyperventilation (causes bilateral hand tingling from respiratory alkalosis)
• Cold exposure (temporary vasoconstriction)
• Tight clothing or shoes compressing nerves

Considerations

• Blood sugar control is the most effective neuropathy prevention and treatment
• B12 supplementation if vegetarian/vegan (1000μg sublingual daily or IM monthly)
• Ergonomic workspace setup for carpal tunnel prevention
• Foot care in diabetic neuropathy (daily inspection, proper footwear, avoid burns)
• Regular exercise improves nerve blood flow and glucose control
• Limit alcohol (>3 units/day causes alcoholic neuropathy)

FAQs.

Red flags.