Case Presentation:

A 24 year old woman with a history of hypothyroidism presented with six months of burning pain and numbness in her feet. These symptoms began as acute pain in her right foot which progressed to both feet. Over the next few months, she developed dizziness, difficulty walking, and a tingling sensation in her hands.

On hospital admission, she was unable to ambulate and had word finding difficulties, urinary retention, and constipation. She was alert and oriented but slow in her responses. She had spastic hyperreflexia in both upper and lower extremities, symmetrically decreased strength, sensation, and proprioception in the lower extremities, a wide‐based stance, and inability to stand without assistance. Basic metabolic panel and liver function tests were normal. Her hemoglobin was 11.4, MCV 124, and ESR 35. TSH was markedly elevated at 20.5, folate level was normal, and vitamin B12 level was > 1200 (normal 211‐911).

She was started on thyroid replacement therapy. Vitamin B12 deficiency was strongly suspected based on her clinical presentation and very high MCV. However, given the high B12 level, alternate diagnoses were investigated. MRI of the brain and spinal cord demonstrated findings consistent with metabolic abnormalities. CSF was normal without oligoclonal bands. Peripheral blood smear showed macrocytosis without megaloblastic features. Iron studies, HIV serology, ANA, SPEP, RPR, and heavy metal screen were all normal or negative. EMG revealed sensorimotor polyneuropathy, multifocal lower extremity partial denervation, and an upper motor neuron recruitment pattern.

A repeat B12 level was very low at <100. Serum homocysteine was at the upper limits of normal and methylmalonic acid was markedly elevated. The patient was started on B12 injections.


Vitamin B12 deficiency, more common in the elderly, can manifest as a characteristic neurologic syndrome called subacute combined degeneration. Patients may develop sensory deficits, ataxia, muscle weakness, loss of proprioception, spasticity, and dementia. Timely treatment is essential to reverse neurologic dysfunction.

In this case, the classic neurologic picture combined with severe macrocytosis should have clinched the diagnosis. We contacted the laboratory to investigate the discordant B12 assays. A PSA run on the first serum sample was 12.6, confirming that the blood in question was from a male, while PSAon the second sample was zero. Therefore, the initial B12 analysis was run on the incorrect patient's blood.


This young woman presented with classic B12 deficiency. Diagnosis and treatment were delayed by a non‐confirmatory lab test. Hospitalists may rely too heavily on advanced laboratory and imaging studies due to their easy availability. Clinical history and exam remain should remain the cornerstone of our practice.

Author Disclosure Block:

N. Bagdasarian, None; M. Peters, None; M. Dandu, None.