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Difficulty: EasyHematology and Oncology

A 62-year-old man comes to the physician due to a 3-month history of progressively worsening fatigue, shortness of breath when climbing stairs, and a tingling sensation in both feet. He has a history of type 2 diabetes mellitus, hypertension, and osteoarthritis. His current medications include metformin, lisinopril, and acetaminophen. Vital signs are within normal limits. Physical examination demonstrates marked conjunctival pallor and decreased vibratory sensation in the lower extremities bilaterally. Lung fields are clear to auscultation. There is no jugular venous distention or lower extremity edema. Laboratory studies reveal:

Hemoglobin: 8.8 g/dL
Mean corpuscular volume (MCV): 114 fL
White blood cell count: 4,800/mm³
Platelet count: 140,000/mm³

A peripheral blood smear demonstrates macro-ovalocytes and several neutrophils with six or more nuclear lobes.

Which of the following is the most appropriate next step in the management of this patient?

  1. A
    Initiate intravenous furosemide therapy
  2. B
    Prescribe a course of oral levofloxacin
  3. Order a serum vitamin B12 levelAnswer
  4. D
    Measure thyroid-stimulating hormone and free T4 levels
  5. E
    Discontinue lisinopril and start a different antihypertensive agent

Answer

The most appropriate next step is to order a serum vitamin B12 level to confirm the diagnosis before initiating supplementation.
The correct answer involves ordering a serum vitamin B12 level. The patient presents with classic clinical and laboratory signs of megaloblastic anemia, which is characterized by a significantly elevated mean corpuscular volume (MCV > 110 fL) and the pathognomonic presence of hypersegmented neutrophils on a peripheral blood smear. Furthermore, the patient exhibits neurologic symptoms, specifically peripheral neuropathy with decreased vibratory sensation, which is a hallmark of vitamin B12 (cobalamin) deficiency and reliably distinguishes it from folate deficiency. The patient is also taking metformin, a medication well-known to interfere with calcium-dependent terminal ileum absorption of the intrinsic factor-vitamin B12 complex. Before initiating targeted replacement therapy, standard medical practice dictates confirming the diagnosis by measuring serum vitamin B12 levels.

Step-by-Step Solution

1
Analyze the clinical presentation and laboratory results.
The patient has a macrocytic anemia (MCV = 114 fL) combined with neurologic symptoms (tingling, decreased vibratory sensation).
Identifying the type of anemia narrows down the differential diagnosis significantly.
2
Interpret the peripheral blood smear findings.
The presence of macro-ovalocytes and hypersegmented neutrophils (> 5 nuclear lobes) is identified.
These findings are highly specific for megaloblastic anemia, which is primarily caused by defective DNA synthesis.
3
Synthesize the findings with the patient's history.
Megaloblastic anemia with dorsal column neurologic signs strongly points to vitamin B12 deficiency. The patient's long-term use of metformin is a known pharmacological risk factor for this deficiency.
Connecting the pathophysiology to the patient's specific medication list establishes the most likely etiology.
4
Determine the most appropriate initial management step.
Ordering a serum vitamin B12 level is chosen as the next step.
Before initiating lifelong or long-term replacement therapy, the suspected deficiency must be formally confirmed via laboratory testing.

Key Concept

Recognition of megaloblastic anemia, its association with vitamin B12 deficiency (often secondary to long-term metformin use), and the appropriate initial diagnostic workup.

Alternative Method

If the initial serum vitamin B12 level returns in the low-normal or borderline range (typically 200-300 pg/mL), measuring serum methylmalonic acid (MMA) is the best alternative method to confirm tissue-level B12 deficiency, as MMA will be consistently elevated.
Estimated Time:1m 0s
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Topics

Hematology and Oncology

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