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Difficulty: MediumHematology and Oncology

A 72-year-old woman is brought to the clinic by her daughter due to progressive forgetfulness, irritability, and unsteadiness on her feet over the past 6 months. Her past medical history is significant for hypothyroidism and type 2 diabetes mellitus, which have been managed with levothyroxine and metformin for the last 8 years. Vital signs are within normal limits. Neurological examination reveals a broad-based gait, decreased vibratory sensation in the distal lower extremities bilaterally, and a positive Romberg sign. Deep tendon reflexes are 1+ at the ankles and 2+ at the patellae. Laboratory studies show:

Hemoglobin: 10.2 g/dL
Mean corpuscular volume (MCV): 108 fL
Platelets: 140,000/mm³
Serum vitamin B12: 245 pg/mL (normal: 200-900 pg/mL)
Serum folate: 14 ng/mL (normal: > 4.0 ng/mL)
Thyroid-stimulating hormone (TSH): 2.4 µU/mL

Which of the following is the most appropriate next step in the diagnosis of this patient's condition?

  1. Measurement of serum methylmalonic acid levelAnswer
  2. B
    Measurement of serum homocysteine level
  3. C
    Testing for anti-intrinsic factor antibodies
  4. D
    Review of a peripheral blood smear
  5. E
    Bone marrow aspiration and biopsy

Answer

Measurement of serum methylmalonic acid level is the most appropriate next step to confirm tissue-level vitamin B12 deficiency in a patient with borderline serum levels.
The patient's presentation of macrocytic anemia, cognitive impairment, and dorsal column signs (decreased vibratory sense, positive Romberg sign) strongly suggests vitamin B12 (cobalamin) deficiency. Her prolonged use of metformin is a well-known risk factor for decreased ileal absorption of vitamin B12. When evaluating suspected B12 deficiency, the initial screening test is a serum vitamin B12 level. However, up to 30% of patients with true tissue deficiency may have levels strictly in the borderline or low-normal range (200-300 pg/mL). In such cases, the next best step is to measure serum methylmalonic acid (MMA). Vitamin B12 is a required cofactor for the enzyme methylmalonyl-CoA mutase, which converts methylmalonyl-CoA to succinyl-CoA. When B12 is deficient, this pathway is blocked, leading to an accumulation of MMA. Therefore, an elevated MMA level confirms a tissue-level B12 deficiency with high sensitivity and specificity.

Step-by-Step Solution

1
Analyze the clinical presentation and laboratory findings.
The patient has macrocytic anemia, peripheral neuropathy with dorsal column involvement, and cognitive changes, suggesting vitamin B12 deficiency. Her long-term metformin use is a key risk factor for malabsorption.
Identifying the clinical syndrome and recognizing risk factors guides the diagnostic workup.
2
Interpret the serum vitamin B12 level.
The serum B12 level is 245 pg/mL, which falls into the borderline or low-normal range (200-300 pg/mL).
A borderline level does not definitively confirm or rule out a tissue-level deficiency, necessitating further testing.
3
Select the appropriate confirmatory diagnostic test.
Methylmalonic acid (MMA) testing is chosen because it accumulates exclusively in vitamin B12 deficiency, making it highly sensitive and specific.
Vitamin B12 is an essential cofactor for the conversion of methylmalonyl-CoA to succinyl-CoA. In deficiency states, the substrate MMA builds up.

Key Concept

In patients with clinical manifestations of vitamin B12 deficiency but borderline serum B12 levels (200-300 pg/mL), a serum methylmalonic acid (MMA) level should be obtained to definitively confirm the diagnosis.
Estimated Time:1m 15s
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Topics

Hematology and Oncology

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