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Zorluk: OrtaOrdering Laboratory Tests

A 68-year-old woman presents to the clinic with a 4-month history of progressive fatigue and a "pins and needles" sensation in her feet. Her past medical history is significant for type 2 diabetes mellitus, for which she has been taking metformin for 8 years. Physical examination reveals a smooth, glossy red tongue and decreased vibratory sensation in the bilateral lower extremities. The Romberg test is positive.

Laboratory studies show:
TestResultReference Range
Hemoglobin10.4 g/dL12.0-15.5 g/dL
Mean corpuscular volume112 fL80-100 fL
Serum vitamin B12245 pg/mL200-900 pg/mL
Serum folate14 ng/mL> 4 ng/mL

Which of the following is the most appropriate next step in laboratory evaluation?

  1. A
    Measurement of anti-intrinsic factor antibodies
  2. Measurement of serum methylmalonic acidCevap
  3. C
    Measurement of hemoglobin A1c
  4. D
    Bone marrow aspiration and biopsy
  5. E
    Measurement of serum iron and total iron-binding capacity

Cevap

Measurement of serum methylmalonic acid
The correct answer is measurement of serum methylmalonic acid. This patient has classic symptoms of vitamin B12 deficiency, including macrocytic anemia, glossitis, and subacute combined degeneration (neuropathy and a positive Romberg test). Long-term metformin use is a well-known risk factor for B12 malabsorption. Her serum B12 level is borderline (between 200 and 300 pg/mL). In cases where clinical suspicion is high but the serum level is equivocal, assessing a functional marker of B12 is required. Vitamin B12 is an essential cofactor for the conversion of methylmalonyl-CoA to succinyl-CoA. Therefore, in true tissue-level B12 deficiency, serum methylmalonic acid (MMA) accumulates and is elevated. This test will definitively establish the diagnosis.

Adım Adım Çözüm

1
Analyze the patient's clinical presentation.
Symptoms of macrocytic anemia, glossitis, and peripheral sensory neuropathy point strongly toward vitamin B12 deficiency, with chronic metformin use acting as a major risk factor.
To establish a pre-test probability before interpreting laboratory results.
2
Interpret the initial laboratory findings.
The patient has macrocytic anemia with a borderline serum vitamin B12 level (200-300 pg/mL) and a normal folate level.
A borderline B12 level does not definitively rule in or rule out a clinical deficiency.
3
Determine the appropriate diagnostic test to confirm tissue-level deficiency.
Order a serum methylmalonic acid (MMA) level, which will be elevated in true B12 deficiency.
MMA is a precursor that requires B12 as a cofactor for metabolism; its accumulation confirms functional deficiency at the cellular level.

Anahtar Kavram

In patients with high clinical suspicion for vitamin B12 deficiency but borderline initial serum B12 levels (200-300 pg/mL), measurement of serum methylmalonic acid (MMA) is the most appropriate next step to confirm the diagnosis.
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Konular

Ordering Laboratory Tests

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